#include <bits/stdc++.h>
using namespace std;
template <class T, class U>
inline void smin(T &a, U b) {
  if (a > b) a = b;
}
template <class T, class U>
inline void smax(T &a, U b) {
  if (a < b) a = b;
}
template <class T>
inline void gn(T &x) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-' && c != -1)
    ;
  if (c == -1) return;
  if (c == '-') sg = 1, c = getchar();
  for (x = 0; c >= '0' && c <= '9';
       x = (x << 1) + (x << 3) + c - '0', c = getchar())
    ;
  if (sg) x = -x;
}
template <class T>
void print(T x) {
  if (x < 0) return putchar('-'), print(-x);
  if (x < 10) {
    putchar(x + '0');
    return;
  }
  print(x / 10);
  putchar(x % 10 + '0');
}
template <class U, class V>
inline void gn(U &u, V &v) {
  gn(u);
  gn(v);
}
template <class U, class V, class W>
inline void gn(U &u, V &v, W &w) {
  gn(u);
  gn(v, w);
}
template <class T>
void println(T x) {
  print(x);
  putchar('\n');
}
template <class T>
void printsp(T x) {
  print(x);
  putchar(' ');
}
template <class U, class V>
inline void print(U u, V v) {
  printsp(u);
  println(v);
}
template <class U, class V, class W>
inline void print(U u, V v, W w) {
  printsp(u);
  print(v, w);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; a = (long long)a * a % m, b >>= 1)
    if (b & 1) ans = (long long)ans * a % m;
  return ans;
}
long long mask[1 << 20];
char s[55][55];
int c[55][55];
double p[55];
void init(int n) {
  for (int i = 0; i <= n; i++) {
    for (int j = 0; j <= i; j++) {
      if (j == 0 || j == i)
        c[i][j] = 1;
      else
        c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
    }
  }
  for (int i = 0; i <= n; i++) p[i] = 1.0 / c[n][i];
}
int main() {
  int n;
  gn(n);
  for (int i = 0; i < n; i++) scanf("%s", s[i]);
  int nn = strlen(s[0]);
  init(nn);
  if (n == 1) {
    puts("0.00000000");
    return 0;
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (i == j) continue;
      int tot = 0;
      for (int k = 0; k < nn; k++)
        if (s[i][k] == s[j][k]) tot += (1 << k);
      mask[tot] |= (1ll << i);
      mask[tot] |= (1ll << j);
    }
  }
  for (int i = (1 << nn); --i;) {
    for (int j = 0; j < nn; j++) {
      if (i & (1 << j)) {
        mask[i ^ (1 << j)] |= mask[i];
      }
    }
  }
  for (int i = 0; i < n; i++) mask[0] |= (1ll << i);
  double ans = 0;
  for (int i = 1; i < (1 << nn); i++) {
    int cnt = (__builtin_popcount(i));
    for (int j = 0; j < nn; j++) {
      if (i & (1 << j)) {
        long long tmp = mask[i] ^ mask[i ^ (1 << j)];
        ans +=
            p[cnt - 1] * (__builtin_popcountll(tmp)) / (nn - (cnt - 1)) * cnt;
      }
    }
  }
  cout << fixed << setprecision(20) << ans / n << endl;
  return 0;
}
