#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, U b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, U b) {
  if (a < b) a = b;
}
template <typename T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T, class T1>
inline void gn(T &first, T1 &second) {
  gn(first);
  gn(second);
}
template <class T, class T1, class T2>
inline void gn(T &first, T1 &second, T2 &z) {
  gn(first);
  gn(second);
  gn(z);
}
template <typename T>
inline void print(T first) {
  if (first < 0) {
    putchar('-');
    return print(-first);
  }
  if (first < 10) {
    putchar('0' + first);
    return;
  }
  print(first / 10);
  putchar(first % 10 + '0');
}
template <typename T>
inline void println(T first) {
  print(first), putchar('\n');
}
template <typename T>
inline void printsp(T first) {
  print(first), putchar(' ');
}
template <class T, class T1>
inline void print(T first, T1 second) {
  printsp(first), println(second);
}
template <class T, class T1, class T2>
inline void print(T first, T1 second, T2 z) {
  printsp(first), printsp(second), println(z);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = 1LL * a * a % m)
    if (b & 1) ans = 1LL * ans * a % m;
  return ans;
}
char s[55][55];
long long mask[1 << 20];
double C[55][55];
int main() {
  int n;
  gn(n);
  if (n == 1) return puts("0.0"), 0;
  for (int i = 0; i < n; i++) scanf("%s", s[i]);
  int m = strlen(s[0]);
  for (int i = 0; i < n; i++)
    for (int j = i + 1; j < n; j++) {
      int val = 0;
      for (int k = 0; k < m; k++) {
        if (s[i][k] == s[j][k]) val |= (1 << k);
      }
      mask[val] |= (1LL << i);
      mask[val] |= (1LL << j);
    }
  int all = 1 << m;
  for (int i = all; --i;) {
    for (int j = 0; j < m; j++)
      if ((((i) >> (j)) & 1)) mask[i ^ (1 << j)] |= mask[i];
  }
  double ans = 0;
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= i; j++) {
      if (i == 0 || i == j)
        C[i][j] = 1;
      else
        C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
    }
  for (int i = 0; i <= m; i++)
    for (int j = 0; j <= i; j++) C[i][j] = 1 / C[i][j];
  mask[0] = (1LL << n) - 1;
  for (int i = 0; i < all; i++) {
    for (int j = 0; j < m; j++) {
      if (!(((i) >> (j)) & 1)) {
        int ii = i | (1 << j);
        int known = (__builtin_popcountll(mask[ii] ^ mask[i]));
        ans += C[m][(__builtin_popcount(i))] / (m - (__builtin_popcount(i))) *
               known * (__builtin_popcount(ii));
      }
    }
  }
  printf("%.15lf\n", ans / n);
  return 0;
}
