#include <bits/stdc++.h>
using namespace std;
#define all(a) a.begin(),a.end()
using ll=long long;

int n=22,m=50;
vector<string> s(n);
bool func(int u){
  //使った集合がuの時に一意に定まるか
  string t=string(m,'o');int o=m;
  for(int i=0;i<n;i++){
    if(!(u&(1<<i)))continue;
    for(int j=0;j<m;j++)
      if(s[i][j]=='x'&&t[j]=='o')t[j]='x',o--;
    if(o==1)return 1;
  }
  return 0;
}

int main(){
  cin>>n>>m;
  for(int i=0;i<n;i++)cin>>s[i];
  vector<long double> dp(1<<n);long double ans=0;
  dp[0]=1;
  for(int i=0;i<(1<<n);i++){
    if(func(i)){
      ans+=dp[i]*__builtin_popcount(i);
      continue;
    }
    for(int j=0;j<n;j++){
      if(i&(1<<j))continue;
      dp[i+(1<<j)]+=dp[i]/(n-__builtin_popcount(i));
    }
  }
  cout<<fixed<<setprecision(15);
  cout<<ans<<endl;
}