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#include <bits/stdc++.h>
using namespace std;

const int MAX_N = 18;
const int MAX_L = 100000;

int powers[MAX_N + 1];

int next_free[MAX_N][MAX_L + 1];
int next_available[MAX_N][MAX_L + 1];

int dp_whole[1<<MAX_N];
int dp_fractional[1<<MAX_N];
string chores[MAX_N];
int L;
int N;

int whole_part;
int frac_a;
int frac_b;
int ceiling;

vector<pair<int, int> > fractions;

void make_next_free(){
	for(int i = 0; i < N; i++){
		next_free[i][L] = L;
		for(int j = L - 1; j >= 0; j--){
			if(chores[i][j] == '.'){
				next_free[i][j] = next_free[i][j + 1];
			} else {
				next_free[i][j] = j;
			}
		}
	}
}

int gcd(int a, int b){
	if(a == 0){
		return b;
	} else if(b == 0){
		return a;
	}
	if(a < b) swap(a, b);
	return gcd(a%b, b);
}

void create_fractions(){
	for(int i = 1; i <= N; i++){
		for(int j = 0; j < i; j++){
			int a = j;
			int b = i;
			int d = gcd(a, b);
			a /= d;
			b /= d;

			fractions.push_back(make_pair(a, b));
		}
	}

	sort(fractions.begin(), fractions.end(),
			[](pair<int, int> P, pair<int, int> Q){return P.first * Q.second < P.second * Q.first;});
	fractions.erase(unique(fractions.begin(), fractions.end()), fractions.end());
}

void find_next_available(){
	for(int i = 0; i < N; i++){
		next_available[i][L] = L + 1;
		for(int j = L - 1; j >= 0; j--){
			if(next_free[i][j] >= j + ceiling){
				next_available[i][j] = j;
			} else {
				next_available[i][j] = next_available[i][j + 1];
			}
		}
	}
}

// we always check a non-empty sleeping schedule.
bool current_works(){
	find_next_available();
	/*
	cout << "NEXT FREE" << endl;
	for(int i = 0; i < N; i++){
		for(int j = 0; j <= L; j++){
			cout << next_free[i][j] << ' ';
		}
		cout << endl;
	}
	cout << "NEXT AVAILABLE" << endl;
	for(int i = 0; i < N; i++){
		for(int j = 0; j <= L; j++){
			cout << next_available[i][j] << ' ';
		}
		cout << endl;
	}
	cout << "DP START" << endl;
	*/

	// for dp, we keep dp_fractional in range [0, frac_b - 1].
	dp_whole[0] = 0;
	dp_fractional[0] = 0;
	bitset<MAX_N> it;
	uint new_index;
	int new_whole, new_fractional;

	for(uint index = 1; index < powers[N]; index++){
		it = bitset<MAX_N>(index);
		dp_whole[index] = L;
		dp_fractional[index] = 1;

		for(int i = 0; i < N; i++){
			if(it[i]){
				new_index = index - powers[i];
				if(dp_whole[new_index] >= L) continue;

				// new_fractional must lie in interval [1, frac_b].
				new_whole = dp_whole[new_index] + whole_part;
				new_fractional = dp_fractional[new_index] + frac_a;
				if(new_fractional > frac_b){
					new_whole++;
					new_fractional -= frac_b;
				} else if(new_fractional == 0){
					new_whole--;
					new_fractional = frac_b;
				}

				if(new_whole >= L) continue;
				
				if(next_free[i][dp_whole[new_index]] <= new_whole){
					// in this case, we can not place sleeping time right in this moment
					// so we update with: (next_available[i][dp_whole[new_index]], 0)
					if(next_available[i][dp_whole[new_index]] <= dp_whole[index]){
						dp_whole[index] = next_available[i][dp_whole[new_index]];
						dp_fractional[index] = 0;
					}
				} else {
					// otherwise, we find that everything works!
					if(new_fractional == frac_b){
						new_fractional = 0;
						new_whole++;
					}
					// so we update with: (new_whole, new_fractional)
					if(new_whole < dp_whole[index]){
						dp_whole[index] = new_whole;
						dp_fractional[index] = new_fractional;
					} else if(new_whole == dp_whole[index]){
						dp_fractional[index] = min(new_fractional, dp_fractional[index]);
					}
				}
			}
		}

		// cout << index << ": " << dp_whole[index] << " + " << dp_fractional[index] << "/" << frac_b << endl;
	}

	if(dp_whole[powers[N] - 1] < L) return true;
	if(dp_whole[powers[N] - 1] == L && dp_fractional[powers[N] - 1] == 0) return true;
	return false;
}

void get_fraction_from_order(int ord){
	whole_part = ord / fractions.size();
	frac_a = fractions[ord - whole_part*fractions.size()].first;
	frac_b = fractions[ord - whole_part*fractions.size()].second;
	if(frac_a > 0){
		ceiling = whole_part + 1;
	} else {
		ceiling = whole_part;
	}
}

bool preprocess(){
	// basically, if there exists a column that is fully
	// occupied, then output is -1. Furthermore, if a
	// column has only 1 person available, then this person
	// must take care of the baby, hence is occupied.
	// Otherwise, if at least 2 people are free at a given
	// slot, then there will always be someone to take
	// care of the baby!

	for(int j = 0; j < L; j++){
		int count = 0;
		for(int i = 0; i < N; i++){
			if(chores[i][j] == '.'){
				count++;
			}
		}

		if(count == 0){
			// baby is left alone
			return false;
		} else if(count == 1){
			for(int i = 0; i < N; i++){
				chores[i][j] = 'X';
			}
		}
	}

	return true;
}

void process(){
	int order_a = 0;
	int order_b = (L + N - 1) / N;
	order_b *= fractions.size();

	while(order_a < order_b){
		int h = (order_a + order_b + 1)/2;

		get_fraction_from_order(h);
		// cout << "checking: " << whole_part << "+" << frac_a << "/" << frac_b << endl;

		if(current_works()){
			order_a = h;
		} else {
			order_b = h - 1;
		}
	}

	get_fraction_from_order(order_a);

	// and here is the output.
	cout << (whole_part * frac_b + frac_a) << "/" << frac_b << endl;
}

int main(){
	cin.tie(NULL);
	ios_base::sync_with_stdio(0);

	cin >> N >> L;
	for(int i = 0; i < N; i++){
		cin >> chores[i];
	}
	
	powers[0] = 1;
	for(int i = 1; i <= N; i++){
		powers[i] = 2*powers[i - 1];
	}

	if(preprocess()){
		make_next_free();
		create_fractions();
		process();
	} else {
		cout << "-1" << endl;
	}
}