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

using ll = int64_t;
using ul = uint64_t;
using ld = long double;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vs = vector<string>;
using vb = vector<bool>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vvd = vector<vd>;
using vvc = vector<vc>;
using vvb = vector<vb>;
using vvl = vector<vl>;
using mint = modint998244353;
using vm = vector<mint>;

int main() {
  int N,M;
  cin >> N >> M;
  unordered_set<ll> b2;
  ll p = 1;
  for (int i = 0; i < 50; i++) {
    b2.insert(p);
    p *= 2;
  }
  vs S(N);
  vl code(N, 0);
  for (int i = 0; i < N; i++) {
    cin >> S[i];
    ll power = 1;
    for (int j = 0; j < M; j++) {
      if (S[i][j] == 'o') code[i] += power;
      power *= 2;
    }
  }
  vector<ld> dp((1 << N), 0);
  vi cnt((1 << N), 0);
  vb end((1 << N), false);
  for (int i = 1; i < (1 << N); i++) {
    ll res = -1,power = 1,now = 0;
    for (int j = 0; j < N; j++) {
      if ((i & power) > 0) {
        if (res == -1) res = code[j];
        else res = (res & code[j]);
        now++;
      }
      power *= 2;
    }
    cnt[i] = now;
    if (b2.count(res)) end[i] = true;
  }
  dp[0] = 1;
  ld ans = 0;
  for (int i = 1; i < (1 << N); i++) {
    ll power = 1;
    for (int j = 0; j < N; j++) {
      if ((i & power) == 0) {
        power *= 2;
        continue;
      }
      if (!end[(i ^ power)]) dp[i] += dp[(i ^ power)]/ld(N - cnt[(i ^ power)]);
      power *= 2;
    }
    if (end[i]) ans += dp[i]*cnt[i];
  }
  cout << fixed << setprecision(9) << ans << endl;
  return 0;
}