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

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

int main() {
	int N, M;
	cin >> N >> M;
	vector<string> S(N);
	rep(i, N) cin >> S[i];
	
	vector<ll> v(N, 0);
	rep(i, N) rep(j, M) if(S[i][j] == 'o') v[i] |= (1LL << j);
	
	double ans = 0;
	vector<double> dp(1 << N, 0);
	dp[0] = 1;
	rep(bit, 1 << N) {
		int pcnt = __builtin_popcount(bit);
		
		ll vand = (1LL << M) - 1;
		rep(i, N) if(bit >> i & 1) vand &= v[i];
		if(__builtin_popcountll(vand) == 1) {
			ans += dp[bit] * pcnt;
			continue;
		}
		
		rep(i, N) {
			if(bit >> i & 1) continue;
			int nbit = bit | (1 << i);
			dp[nbit] += dp[bit] / (N - pcnt);
		}
	}
	
	cout << fixed << setprecision(15) << ans << "\n";
}