#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() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector<ll> a(n, 0);
    rep(i, n) {
        string s;
        cin >> s;
        rep(j, m) {
            if (s[j] == 'o')
                a[i] += 1LL << j;
        }
    }
    vector<double> dp(1 << n, -1);
    function<double(int)> rec = [&](int i) {
        ll x = (1LL << 60) - 1;
        if (dp[i] != -1)
            return dp[i];
        rep(j, n) {
            if (i >> j & 1) {
                x &= a[j];
            }
        }
        int bi = 0;
        rep(j, m) if (x >> j & 1) bi++;
        if (bi == 1) {
            return dp[i] = 0;
        }
        int cnt = 0;
        double ans = 0;
        rep(j, n) {
            if (i >> j & 1)
                continue;
            ans += rec(i ^ 1 << j);
            cnt++;
        }
        return dp[i] = ans / cnt + 1;
    };
    cout << fixed << setprecision(12);
    cout << rec(0) << endl;
    return 0;
}