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

#define rep(i, n) for(int i = 0; i < (int)(n); ++i)

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	
	int N, M;
	cin >> N >> M;
	vector<string> S(N);
	rep(i, N) cin >> S[i];
	
	double ans = 0;
	vector<double> dp(1 << N, 0);
	dp[0] = 1;
	
	rep(bit, 1 << N) {
		vector<bool> vec(M, 1);
		rep(i, N) if(bit >> i & 1) rep(j, M) if(S[i][j] == 'x') vec[j] = 0;
		int cnt = 0;
		rep(i, M) if(vec[i]) cnt += 1;
		if(cnt == 1) {
			ans += __builtin_popcount(bit) * dp[bit];
			continue;
		}
		
		dp[bit] /= (N - __builtin_popcount(bit));
		rep(i, N) {
			if(bit >> i & 1) continue;
			dp[bit | (1 << i)] += dp[bit];
		}
	}
	
	cout << fixed << setprecision(15) << ans << "\n";
}