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

void solve(){
  using ull=unsigned long long;
  int n,m;
  cin>>n>>m;
  vector<ull> a(n);
  vector<ull> b(1<<n);
  for (int i=0;i<n;i++){
    string s;
    cin>>s;
    for (int j=0;j<m;j++){
      a[i]+=(1<<j)*(s[j]=='o');
    }
    b[1<<i]=a[i];
  }
  b[0]=(1<<m)-1;
  vector<double> dp(1<<n);
  dp[0]=1;
  double ans=0;
  for (int bit=0;bit<1<<n;bit++){
    ull mb=bit&(-bit);
    if (bit) b[bit]=b[bit-mb]&b[mb];
    if (__builtin_popcount(b[bit])==1){
      ans+=dp[bit]*__builtin_popcount(bit);
      continue;
    }
    int p=n-__builtin_popcount(bit);
    for (int i=0;i<n;i++){
      if (bit>>i&1) ;
      else dp[bit+(1<<i)]+=dp[bit]/p;
    }
  }
  cout<<fixed<<setprecision(10);
  cout<<ans<<endl;
}

int main(){
  int t=1;
  //cin>>t;
  while (t--) solve();
}