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#include<bits/stdc++.h>
using namespace std;
using LL=long long;
#define FOR(i,l,r)for(int i=(l);i<=(r);++i)
#define REP(i,n)FOR(i,0,(n)-1)
#define ssize(x)int(x.size())
#ifdef DEBUG
auto operator<<(auto&o,auto x)->decltype(x.end(),o);
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto&operator<<(auto&o,tuple<auto,auto,auto>t){return o<<"("<<get<0>(t)<<", "<<get<1>(t)<<", "<<get<2>(t)<<")";}
auto&operator<<(auto&o,tuple<auto,auto,auto,auto>t){return o<<"("<<get<0>(t)<<", "<<get<1>(t)<<", "<<get<2>(t)<<", "<<get<3>(t)<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif

/*
 * Opis: O(\log max\_val), szuka największego \texttt{a/b}, że \texttt{is\_ok(a/b)} oraz \texttt{0 <= a,b <= max\_value}.
 * Zakłada, że \texttt{is\_ok(0) == true}.
 */
using Frac = pair<LL, LL>;
Frac my_max(Frac l, Frac r) {
	return l.first * __int128_t(r.second) > r.first * __int128_t(l.second) ? l : r;
}
Frac binsearch(LL max_value, function<bool (Frac)> is_ok) {
	assert(is_ok(pair(0, 1)) == true);
	Frac left = {0, 1}, right = {1, 0}, best_found = left;
	int current_dir = 0;
	while(max(left.first, left.second) <= max_value) {
		best_found = my_max(best_found, left);
		auto get_frac = [&](LL mul) {
			LL mull = current_dir ? 1 : mul;
			LL mulr = current_dir ? mul : 1;
			return pair(left.first * mull + right.first * mulr, left.second * mull + right.second * mulr);
		};
		auto is_good_mul = [&](LL mul) {
			Frac mid = get_frac(mul);
			return max(mid.first, mid.second) <= max_value and is_ok(mid) == current_dir;
		};
		LL power = 1;
		for(; is_good_mul(power); power *= 2) {}
		LL bl = power / 2 + 1, br = power;
		while(bl != br) {
			LL bm = (bl + br) / 2;
			if(not is_good_mul(bm))
				br = bm;
			else
				bl = bm + 1;
		}
		tie(left, right) = pair(get_frac(bl - 1), get_frac(bl));
		if(current_dir == 0)
			swap(left, right);
		current_dir ^= 1;
	}
	return best_found;
}
Frac to_frac(LL x) {
	return {x, 1};
}
Frac operator+(Frac a, Frac b) {
	auto [xa, ya] = a;
	auto [xb, yb] = b;
	if (ya != yb) {
		return {xa * yb + xb * ya, ya * yb};
	}
	else {
		return {xa + xb, ya};
	}
}
Frac operator-(Frac a, Frac b) {
	auto [xa, ya] = a;
	auto [xb, yb] = b;
	if (ya != yb) {
		return {xa * yb - xb * ya, ya * yb};
	}
	else {
		return {xa - xb, ya};
	}
}
int floor(Frac a) {
	auto [x, y] = a;
	return int(x / y);
}
int ceil(Frac a) {
	auto [x, y] = a;
	return int((x + y - 1) / y);
}
bool lt(Frac a, Frac b) {
	return (a - b).first < 0;
}
bool leq(Frac a, Frac b) {
	return (a - b).first <= 0;
}
bool eq(Frac a, Frac b) {
	return (a - b).first == 0;
}

using T = pair<int, int>;
struct Node {
	T sum = {0, 0};
	int lazy = 0;
	int sz = 1;
};
void push_to_sons(Node &n, Node &l, Node &r) {
	auto push_to_son = [&](Node &c) {
		c.sum.first += n.lazy;
		c.lazy += n.lazy;
	};
	push_to_son(l);
	push_to_son(r);
	n.lazy = 0;
}
Node merge(Node l, Node r) {
	return Node{
		.sum = min(l.sum, r.sum),
		.lazy = 0,
		.sz = l.sz + r.sz
	};
}
void add_to_base(Node &n, int val) {
	n.sum.first += val;
	n.lazy += val;
}
struct Tree {
	vector<Node> tree;
	int sz = 1;
	Tree(int n, const vector<int>& initial) {
		while(sz < n)
			sz *= 2;
		tree.resize(sz * 2);
		const int len = min(sz, ssize(initial));
		REP(i, len) {
			tree[i + sz].sum = {initial[i], -i};
		}
		for(int v = sz - 1; v >= 1; v--)
			tree[v] = merge(tree[2 * v], tree[2 * v + 1]);
	}
	void push(int v) {
		push_to_sons(tree[v], tree[2 * v], tree[2 * v + 1]);
	}
	Node get(int l, int r, int v = 1) {
		if(l == 0 and r == tree[v].sz - 1)
			return tree[v];
		push(v);
		int m = tree[v].sz / 2;
		if(r < m)
			return get(l, r, 2 * v);
		else if(m <= l)
			return get(l - m, r - m, 2 * v + 1);
		else
			return merge(get(l, m - 1, 2 * v), get(0, r - m, 2 * v + 1));
	}
	void update(int l, int r, int val, int v = 1) {
		if(l == 0 && r == tree[v].sz - 1) {
			add_to_base(tree[v], val);
			return;
		}
		push(v);
		int m = tree[v].sz / 2;
		if(r < m)
			update(l, r, val, 2 * v);
		else if(m <= l)
			update(l - m, r - m, val, 2 * v + 1);
		else {
			update(l, m - 1, val, 2 * v);
			update(0, r - m, val, 2 * v + 1);
		}
		tree[v] = merge(tree[2 * v], tree[2 * v + 1]);
	}
};

/*
 * Opis: Kolejka wspierająca dowolną operację łączną, O(1) zamortyzowany.
 *   Konstruktor przyjmuje dwuargumentową funkcję oraz jej element neutralny.
 *   Dla minów jest \texttt{AssocQueue<int> q([](int a, int b)\{ return min(a, b); \}, numeric\_limits<int>::max());}
 */
template<typename T>
struct AssocQueue {
	using fn = function<T(T, T)>;
	fn f;
	vector<pair<T, T>> s1, s2; // {x, f(pref)}
	AssocQueue(fn _f, T e = T()) : f(_f), s1({{e, e}}), s2({{e, e}}) {}
	void mv() {
		if (ssize(s2) == 1)
			while (ssize(s1) > 1) {
				s2.emplace_back(s1.back().first, f(s1.back().first, s2.back().second));
				s1.pop_back();
			}
	}
	void emplace(T x) {
		s1.emplace_back(x, f(s1.back().second, x));
	}
	void pop() {
		mv();
		s2.pop_back();
	}
	T calc() {
		return f(s2.back().second, s1.back().second);
	}
	T front() {
		mv();
		return s2.back().first;
	}
	int size() {
		return ssize(s1) + ssize(s2) - 2;
	}
	void clear() {
		s1.resize(1);
		s2.resize(1);
	}
};

void solve() {
	int n, L;
	cin >> n >> L;
	vector occupied(n, vector (L, false));
	REP(i, n) {
		string s;
		cin >> s;
		REP(j, L)
			occupied[i][j] = s[j] == 'X';
	}
	debug(n, L, occupied);

	REP(i, L) {
		int sum = 0;
		REP(j, n) {
			sum += occupied[j][i];
		}
		if (sum == n) {
			cout << -1 << '\n';
			return;
		}
	}

	auto adjust_frac = [&](Frac frac) -> Frac {
		auto [a, b] = frac;
		if (b <= n)
			return frac;
		Frac best = to_frac(L + 1);
		FOR(k, 1, n) {
			LL x = (a * k + b - 1) / b;
			best = min(best, Frac{x, k}, lt);
		}
		//debug(frac, best);
		return best;
	};

	vector initial_cnt(n * L, 0);
	vector initial_can_sleep(n * L, false);

	map<Frac, bool> is_okay_cache;
	auto is_okay = [&](Frac frac) -> bool {
		if (frac.first == 0)
			return true;
		frac = adjust_frac(frac);
		{
			auto it = is_okay_cache.find(frac);
			if (it != is_okay_cache.end())
				return it->second;
		}
		const auto hash = frac;

		debug(frac);
		const int length = int(frac.first);
		const int factor = int(frac.second);
		const int size = L * int(factor);

		REP(i, L) {
			int sum = 0;
			REP(j, n)
				sum += not occupied[j][i];
			REP(j, factor)
				initial_cnt[i * factor + j] = sum;
		}
		debug(initial_cnt);

		Tree tree(size, initial_cnt);

		// return {can, blocker}
		auto can_sleep_here = [&](int position) -> pair<bool, int> {
			if (position + length > size)
				return {false, size};
			const auto [value, id] = tree.get(position, position + length - 1).sum;
			if (value > 1)
				return {true, -1};
			else
				return {false, -id};
		};

		auto get_elem = [&](int position) {
			if (position < size)
				return initial_cnt[position];
			return 0;
		};
		AssocQueue<int> q([](int a, int b){ return min(a, b); }, numeric_limits<int>::max());
		{
			const int len = min(length, size);
			REP(i, len) {
				q.emplace(get_elem(i));
			}
		}
		REP(i, size) {
			initial_can_sleep[i] = q.calc();
			q.emplace(get_elem(i + length));
			q.pop();
		}
		debug(initial_can_sleep);

		vector initial_can_sleep_compressed(L, -1);
		REP(i, size) {
			if (initial_can_sleep[i])
				initial_can_sleep_compressed[i / factor] = i;
		}

		vector options(n, vector<int>{});
		REP(i, n) {
			int last_occupied = size;
			for (int j = L - 1; j >= 0; --j) {
				if (occupied[i][j])
					last_occupied = j * factor;
				if (initial_can_sleep[j] == -1)
					continue;
				const int bound = min(last_occupied - length, initial_can_sleep_compressed[j]);
				if (bound >= j * factor)
					options[i].emplace_back(bound);
			}
			reverse(options[i].begin(), options[i].end());
		}
		debug(options);

		auto find_first_good = [&](int id) -> int {
			int last_obstacle = -1;
			while (true) {
				auto it = upper_bound(options[id].begin(), options[id].end(), last_obstacle);
				if (it == options[id].end())
					return -1;
				const int position = max((*it) / factor * factor, last_obstacle + 1);
				const auto [value, obstacle] = can_sleep_here(position);
				if (value)
					return position;
				last_obstacle = obstacle;
			}
		};
		auto mark_as_sleep = [&](int position) {
			tree.update(position, position + length - 1, -1);
		};
		auto unmark_as_sleep = [&](int position) {
			tree.update(position, position + length - 1, 1);
		};
		using Last = pair<int, int>;
		auto rec = [&](auto&& self, int mask, Last last) -> bool {
			if (mask == 0)
				return true;
			vector<Last> moves;
			REP(i, n) {
				if (((mask >> i) & 1) == 0)
					continue;
				auto my_time = find_first_good(i);
				if (my_time == -1)
					return false;
				moves.emplace_back(my_time, i);
			}
			sort(moves.begin(), moves.end());
			while (ssize(moves) > 2) {
				moves.pop_back();
			}
			for (const auto& [my_time, id] : moves) {
				if (Last{my_time, id} < last)
					continue;
				mark_as_sleep(my_time);
				const int new_mask = mask ^ (1 << id);
				if (self(self, new_mask, Last{my_time, id}))
					return true;
				unmark_as_sleep(my_time);
			}
			return false;
		};
		const bool ret = rec(rec, (1 << n) - 1, Last{-1, -1});

		is_okay_cache[hash] = ret;
		return ret;
	};

	const int binsearch_limit = max(n, L);
	auto [nom, denom] = binsearch(binsearch_limit, is_okay);
	cout << nom << '/' << denom << '\n';
}

int main() {
	cin.tie(0)->sync_with_stdio(0);

#ifdef TESTS
	int t;
	cin >> t;
	REP(tt, t) {
		if (tt % 10000 == 0)
			cerr << tt << endl;
		solve();
	}
#else
	solve();
#endif
}