#include <bits/stdc++.h>   using namespace std;   const int maxn = 50 + 5, maxL = (1 << 20) + 10;   int N, L;   char s[maxn][25];   bool vis[maxL];   long double F[maxL], R[25];   long long dask[maxL];   bool check(int tt, int k) {     if (dask[tt] & (1LL << k))       return false;     else       return true;   }   long double dfs(int tt, int g) {     if (g == 0) return 0.0;     if (vis[tt]) return F[tt];     long double ret = 0.0;     long double step = L - g + 1;     for (int i = 0; i < L; i++) {       if ((1 << i) & tt) continue;       int nt = (1 << i) | tt;       int wq = __builtin_popcountll(dask[nt] ^ dask[tt]);       ret += step * R[g] * wq;       ret += dfs(nt, g - 1);     }     vis[tt] = true;     F[tt] = ret;     return ret;   }   void Prepare() {     memset(dask, 0, sizeof(dask));     for (int i = (1), _i = (N); i <= _i; i++)       for (int j = (i + 1), _j = (N); j <= _j; j++) {         int same = 0;         for (int k = 0; k < L; k++)           if (s[i][k] == s[j][k]) same |= (1 << k);         dask[same] |= (1LL << j) | (1LL << i);       }     for (int i = (1 << L) - 1; i; i--)       for (int k = 0; k < L; k++)         if (i & (1 << k)) dask[i ^ (1 << k)] |= dask[i];   }   int main() {     for (; scanf( %d , &N) != EOF;) {       for (int i = (1), _i = (N); i <= _i; i++) scanf( %s , s[i]);       L = strlen(s[1]);       int gg = L;       R[L] = 1.0 / (long double)L;       for (int i = (L - 1), _i = (1); i >= _i; i--) {         gg--;         R[i] = R[i + 1] / (long double)gg;       }       memset(vis, 0, sizeof(vis));       long double ans = 0.0;       Prepare();       ans = dfs(0, L);       ans /= (long double)N;       printf( %.15lf n , (double)ans);     }     return 0;   }   