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

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N,M; cin >> N >> M;
    long long n2 = 1<<N,m2 = 1LL<<M;
    vector<double> dp(n2);
    vector<long long> mask(N);
    for(auto &m : mask){
        string s; cin >> s;
        for(int i=0; i<M; i++) if(s.at(i) == 'o') m += 1LL<<i;
    }
    vector<long long> left(n2,m2-1);

    dp.at(0) = 1;
    double answer = 0;
    for(int i=0; i<n2; i++){
        if(i){
            for(int k=0; k<N; k++) if(i&(1<<k)) left.at(i) = left.at(i-(1<<k))&mask.at(k);
        }
        if(__popcount((unsigned long long)left.at(i)) == 1) continue;
        double v = dp.at(i);
        answer += v;
        int div = __popcount((unsigned int)(n2-1-i));
        v /= div;
        for(int k=0; k<N; k++) if(!(i&(1<<k))) dp.at(i+(1<<k)) += v;
    }   


    cout << fixed << setprecision(20) << answer << endl;
}