#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 <class 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 t>
inline void gn(T &first, t &second) {
  gn(first);
  gn(second);
}
template <class T, class t, class t1>
inline void gn(T &first, t &second, t1 &z) {
  gn(first);
  gn(second);
  gn(z);
}
template <class 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 <class T>
inline void printsp(T first) {
  print(first);
  putchar(' ');
}
template <class T>
inline void println(T first) {
  print(first);
  putchar('\n');
}
template <class T, class U>
inline void print(T first, U second) {
  printsp(first);
  println(second);
}
template <class T, class U, class V>
inline void print(T first, U second, V z) {
  printsp(first);
  printsp(second);
  println(z);
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
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;
}
const int N = 2222222, MX = (1 << 17) - 1;
double ans[N];
int dp[1 << 17];
long long dif[N];
char s[55][22];
void solve() {
  int m;
  gn(m);
  for (int i = 0; i < m; i++) scanf("%s", s[i]);
  int n = strlen(s[0]);
  for (int i = 0; i < m; i++) {
    for (int j = i + 1; j < m; j++) {
      int bit = 0;
      for (int k = 0; k < n; k++)
        if (s[i][k] == s[j][k]) bit |= (1 << k);
      dif[bit] |= (1LL << i);
      dif[bit] |= (1LL << j);
    }
  }
  for (int i = 1; i < (1 << 17); i++) dp[i] = dp[i & (i - 1)] + 1;
  for (int b = (1 << n) - 1, p = 0; b >= 0; b--, p = 0) {
    for (int i = 0; i < n; i++)
      if (b & (1 << i)) dif[b ^ (1 << i)] |= dif[b];
    for (int i = 0; i < n; i++)
      if (!(b & (1 << i))) ans[b] += ans[b ^ (1 << i)], p++;
    if (p > 0) ans[b] /= p;
    ans[b] += (dp[dif[b] & MX] + dp[(dif[b] >> 17) & MX] + dp[(dif[b] >> 34)]) /
              (double)m;
  }
  printf("%.11lf\n", ans[0]);
  return;
}
int main() {
  solve();
  return 0;
}
