#include <bits/stdc++.h>
using namespace std;
int main(void){
    long long n,m;
    cin>>n>>m;
    vector<long long>d(n,0);
    string s[n];
    long long ansd=(1LL<<m)-1;
    vector<long double>dp((1<<n),0);
    for(int i=0;i<n;i++){
        cin>>s[i];
        for(long long j=0;j<m;j++){
            d[i]+=(((long long)(s[i][j]=='o'))<<j);
        }
        ansd&=d[i];
    }
    dp[0]=1;
    long double ans=0;
    for(int i=0;i<(1<<n);i++){
        int pcn=0;
        long long tmp=(1LL<<m)-1;
        for(int j=0;j<n;j++){
            if((1<<j)&i){
                tmp&=d[j];
                pcn++;
            }
        }
        if(tmp==ansd){
            ans+=dp[i]*pcn;
            //cout<<"!";
        }else{
            for(int j=0;j<n;j++){
                if((1<<j)&i)continue;
                dp[i|(1<<j)]+=dp[i]/(n-pcn);
            }
        }
        //cout<<i<<" "<<ans<<" "<<dp[i]<<" "<<pcn<<" "<<tmp<<" "<<ansd<<endl;
    }
    cout<<fixed << setprecision(15)<<ans<<endl;
    return 0;
}