#include <bits/stdc++.h>   using namespace std;   int n, m;   char st[55][25];   int f[1100005], dx[1100005];   long long ms[1100005];   double dp[1100005], cf[1100005];   int main() {     int i, j, k, sum, s;     double ans = 0, t;     cin >> n;     for (i = 1; i <= n; i++) scanf( %s , st[i]);     m = strlen(st[1]);     for (i = 1; i <= n; i++)       for (j = i + 1; j <= n; j++) {         sum = 0;         for (k = 0; k < m; k++)           if (st[i][k] == st[j][k]) sum |= (1 << k);         ms[sum] |= (1ll << i - 1) | (1ll << j - 1);       }     for (s = (1 << m) - 1; s >= 0; s--) {       f[s] = n;       for (k = 0; k < m; k++)         if (s & (1 << k)) dx[s]++;       for (i = 1; i <= n; i++)         if (ms[s] & (1ll << i - 1)) f[s]--;       for (k = 0; k < m; k++)         if (s & (1 << k)) ms[s ^ (1 << k)] |= ms[s];     }     dp[0] = 1;     for (s = 0; s < (1 << m); s++) {       ans += cf[s] * dx[s];       if (f[s] == n) continue;       t = dp[s] / (m - dx[s]) / (n - f[s]);       for (k = 0; k < m; k++)         if (!(s & (1 << k))) {           dp[s | (1 << k)] += t * (n - f[s | (1 << k)]);           cf[s | (1 << k)] += t * (f[s | (1 << k)] - f[s]);         }     }     printf( %.12f n , ans);     return 0;   }   