#include <bits/stdc++.h>   using namespace std;   char s[60][30];   long long id[1 << 20];   int num[1 << 20];   double f[1 << 20];   int main() {     int n;     cin >> n;     for (int i = 1; i <= n; i++) {       scanf(  %s , &s[i]);     }     int m = strlen(s[1]);     for (int i = 1; i <= n; i++) {       for (int j = i + 1; j <= n; j++) {         int mask = 0;         for (int k = 0; k < m; k++) {           mask |= s[i][k] == s[j][k] ? (1ll << k) : 0;         }         id[mask] |= (1ll << i) | (1ll << j);       }     }     for (int mask = (1 << m) - 1; mask >= 0; mask--) {       for (int i = 0; i < m; i++) {         if ((mask >> i) & 1) {           id[mask ^ (1 << i)] |= id[mask];         }       }     }     for (int i = 0; i <= (1 << m) - 1; i++) {       long long tmp = id[i];       int ans = 0;       while (tmp) {         ans += tmp & 1;         tmp >>= 1;       }       num[i] = ans;       if (num[i] == 1) num[i] = 0;     }     f[(1 << m) - 1] = 0;     for (int s = (1 << m) - 2; s >= 0; s--) {       if (!num[s]) {         f[s] = 0;         continue;       }       int tot = 0;       for (int i = 0; i < m; i++)         if (!(s & (1 << i))) tot++;       for (int i = 0; i < m; i++) {         if (((1 << i) & s)) continue;         f[s] += (1.0 * f[s | (1 << i)] / (double)tot * (double)num[s | (1 << i)] /                  (double)num[s]);       }       f[s] += 1.0;     }     printf( %.10lf , f[0]);     return 0;   }   