#include <bits/stdc++.h>
#include <cassert>
using namespace std;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
using ll = long long;
constexpr int inf = 1e9;
signed main() {
    int N,M;
    cin >> N >> M;
    vector<string> S(N);
    rep(i,N) cin >> S[i];
    assert(N <= 6);
    vector<int> per(N);
    ranges::iota(per,0);
    ll cnt = 0;
    do
    {
        string p;
        rep(i,M) p.push_back('o');
        rep(i,N)
        {
            rep(j,N)
            {
                if (S[per[i]][j] == 'x') p[j] = 'x';
            }
            if (ranges::count(p,'o') == 1)
            {
                cnt += i + 1;
                break;
            }
        }
    }while (ranges::next_permutation(per).found);
    ll div = 1;
    for (int i = 1; i <= N; i++) div *= i;
    cout << fixed << setprecision(10) << static_cast<long double>(cnt) / static_cast<long double>(div) << endl;
}