#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
// using namespace atcoder;
// using mint = modint1000000007;
// const int mod = 1000000007;
// using mint = modint998244353;
// const int mod = 998244353;
// const int INF = 1e9;
// const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i, l, r) for (int i = (l); i < (r); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rrep2(i, l, r) for (int i = (r)-1; i >= (l); --i)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define P pair<int, int>
template<typename A, typename B> inline bool chmax(A& a, const B& b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A& a, const B& b) { if (a > b) { a = b; return true; } return false; }
#ifndef KWM_T_BASE_BUILTIN_HPP
#define KWM_T_BASE_BUILTIN_HPP

/**
 * @brief ローカル環境用(MSC)builtin
 */
#ifdef _MSC_VER
#include <intrin.h>
#define __builtin_popcount (int)__popcnt
#define __builtin_popcountll (int)__popcnt64
inline int __builtin_ctz(unsigned int n) { unsigned long i; _BitScanForward(&i, n); return i; }
inline int __builtin_ctzll(unsigned long long n) { unsigned long i; _BitScanForward64(&i, n); return i; }
inline int __builtin_clz(unsigned int n) { unsigned long i; if (_BitScanReverse(&i, n)) { return 31 - i; }return 32; }
inline int __builtin_clzll(unsigned long long n) { unsigned long i; if (_BitScanReverse64(&i, n)) { return 63 - i; }return 64; }
#endif

#endif // KWM_T_BASE_BUILTIN_HPP
int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n, m; cin >> n >> m;
	vector<string>s(n);
	rep(i, n)cin >> s[i];
	double ans = 0;
	vector<double>dp(1 << n, 0);
	dp[0] = 1;
	rep(i, 1 << n) {
		if (dp[i] < 1e-13) continue;

		vector<bool> vec(m, 1);
		rep(j, n) if ((i >> j) & 1) rep(k, m) if (s[j][k] == 'x') vec[k] = 0;
		int cnt = 0;
		rep(j, m) if (vec[j]) cnt += 1;
		if (cnt == 1) {
			ans += __builtin_popcount(i) * dp[i];
			continue;
		}
		dp[i] /= (n - __builtin_popcount(i));
		rep(j, n) {
			if ((i >> j) & 1) continue;
			dp[i | (1 << j)] += dp[i];
		}
	}
	cout << fixed << setprecision(15) << ans << endl;;
	return 0;
}