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// Author: Olaf Surgut (surgutti)
// Created on 12-03-2025 23:55:25
#include "bits/stdc++.h"
using namespace std;

// #define int long long
#define ll long long
#define ld long double

#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define all(x) begin(x),end(x)
#define FOR(i,l,r) for(int i=(l);i<=(r);i++)
#define ROF(i,r,l) for(int i=(r);i>=(l);i--)

auto& operator<<(auto &o, pair<auto, auto> p) {
	return o << "(" << p.st << ", " << p.nd << ")";}
auto operator<<(auto &o, auto x)->decltype(end(x), o) {
	o << "{"; int i=0; for (auto e : x) o << ","+!i++ << e;
	return o << "}";}

#ifdef LOCAL
#define debug(x...) cerr << "[" #x "]: ", [](auto...$) { \
	((cerr << $ << "; "),...) << endl; }(x)
#else
#define debug(...)
#endif

#define rep(i,a,b) for(int i = a; i < (b); i++)
using pii = pair<int, int>;
using vi = vector<int>;

const int N = 18;
const int L = 100000 + 7;

int n, l, m;
string grid[N];
int cnt[L * N];
vi dp[1 << N];

void solve(int tt) {
	cin >> n >> l;

	rep(i, 0, n)
		cin >> grid[i];

	pii res{0, 1};
	ROF(x, n, 1) {
		bool done = false;
		rep(y, x + 1, n) {
			if (y % x == 0) {
				done = true;
			}
		}
		if (done)
			continue;
		
		m = x * l;
		
		rep(j, 0, m) {
			cnt[j] = 0;
			rep(i, 0, n)
				if (grid[i][j / x] == '.')
					cnt[j]++;
		}

		if (*min_element(cnt, cnt + m) == 0) {
			cout << "-1\n";
			return;
		}
		
		auto check = [&](int t) {	
			rep(mask, 1, 1 << n) {
				dp[mask].clear();

				rep(bit, 0, n) if (mask >> bit & 1) {
					auto const& dq = dp[mask ^ (1 << bit)];

					for (int i : dq)
						rep(k, i, i + t)
							cnt[k]--;

					int j = (sz(dq) > 0 ? dq.back() : 0);

					int ile = 0;
					rep(k, j, j + t) {
						if (cnt[k] < 2 || grid[bit][k / x] == 'X')
							ile++;
					}

					while (j + t - 1 < m) {
						if (ile == 0) {
							auto new_dp = dq;
							new_dp.insert(new_dp.begin(), j);

							if (sz(dp[mask]) == 0 || dp[mask] > new_dp)
								dp[mask] = new_dp;	
							break;
						}

						if (sz(dp[mask]) && dp[mask][0] <= j)
							break;

						if (cnt[j] < 2 || grid[bit][j / x] == 'X')
							ile--;
							
						if (cnt[j + t] < 2 || grid[bit][(j + t) / x] == 'X')
							ile++;

						j++;
					}

					for (int i : dq)
						rep(k, i, i + t)
							cnt[k]++;
				}

				if (sz(dp[mask]) == 0)
					return false;	
			}

			return true;
		};

		int left = 0, right = m / 2 + 1;

		while ((left + 1) * res.nd <= res.st * x)
			left++;

		if (res != pii{0, 1}) {
			while ((right - 1) * res.nd >= (res.st + 1) * x)
				right--;
		}
		
		while (right - left > 1) {
			int mid = (left + right) >> 1;
		
			if (check(mid))
				left = mid;
			else
				right = mid;
		}

		if (left * res.nd > res.st * x) {
			res = {left, x};
		}
	}

	int gg = gcd(res.st, res.nd);
	cout << res.st / gg << "/" << res.nd / gg << '\n';
}

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

	int tt = 1;
	// cin >> tt;
	FOR(i, 1, tt) {
		solve(i);
	}

	return 0;
}