#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;

int main(){
    int N, M;
    cin >> N >> M;
    vector<bitset<50>> S(N);
    for (int i = 0; i < N; i++){
        string s;
        cin >> s;
        for (int j = 0; j < M; j++){
            if (s[j] == 'o') S[i][j] = 1;
        }
    }

    int L = 1<<N;
    vector<bitset<50>> C(L);
    C[0].set();
    vector<ll> bad(N+1, 0);

    for (int bit = 0; bit < L; bit++){
        if (bit){
            int b = __builtin_ctz(bit);
            int pre = bit^(1<<b);
            C[bit] = C[pre]&S[b];
        }
        int k = __builtin_popcount(bit);
        if (C[bit].count() >= 2) bad[k]++;
    }

    vector<ld> CC(N+1);
    for (int k = 0; k <= N; k++){
        if (k == 0) CC[0] = 1;
        else CC[k] = CC[k-1]*(N-k+1)/k;
    }

    ld ans = 0;
    for (int k = 0; k < N; k++) ans += (ld)bad[k]/CC[k];
    cout << fixed << setprecision(20) << ans << endl;
}