#include <bits/stdc++.h>
using namespace std;
const int iinf = 1e9 + 7;
const long long linf = 1ll << 60;
const double dinf = 1e60;
template <typename T>
inline void scf(T &x) {
  bool f = 0;
  x = 0;
  char c = getchar();
  while ((c < '0' || c > '9') && c != '-') c = getchar();
  if (c == '-') {
    f = 1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    x = x * 10 + c - '0';
    c = getchar();
  }
  if (f) x = -x;
  return;
}
template <typename T1, typename T2>
void scf(T1 &x, T2 &y) {
  scf(x);
  return scf(y);
}
template <typename T1, typename T2, typename T3>
void scf(T1 &x, T2 &y, T3 &z) {
  scf(x);
  scf(y);
  return scf(z);
}
template <typename T1, typename T2, typename T3, typename T4>
void scf(T1 &x, T2 &y, T3 &z, T4 &w) {
  scf(x);
  scf(y);
  scf(z);
  return scf(w);
}
inline char mygetchar() {
  char c = getchar();
  while (c == ' ' || c == '\n') c = getchar();
  return c;
}
template <typename T>
void chkmax(T &x, const T &y) {
  if (y > x) x = y;
  return;
}
template <typename T>
void chkmin(T &x, const T &y) {
  if (y < x) x = y;
  return;
}
const int N = 55;
const int M = 22;
int n, m;
char str[N][M];
long long c[M][M];
long long dif[1050000];
void MEI() {
  scf(n);
  for (int i = 0; i < (n); ++i) scanf("%s", str[i]);
  m = strlen(str[0]);
  for (int i = 0; i < (n); ++i)
    for (int j = 0; j < (i); ++j) {
      int msk = 0;
      for (int k = 0; k < (m); ++k)
        if (str[i][k] == str[j][k]) msk |= 1 << k;
      dif[msk] |= (1ll << i) | (1ll << j);
    }
  for (int msk = ((1 << m)) - 1; msk >= 0; --msk)
    for (int i = 0; i < (m); ++i)
      if (msk >> i & 1) dif[msk ^ (1 << i)] |= dif[msk];
  c[0][0] = 1ll;
  for (int i = (1); i <= (m); ++i) {
    c[i][0] = c[i][i] = 1ll;
    for (int j = (1); j <= (i - 1); ++j)
      c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
  }
  return;
}
void ZI() {
  double ans = 0.0;
  for (int msk = 0; msk < ((1 << m)); ++msk) {
    ans += 1.0 * __builtin_popcountll(dif[msk]) /
           (1.0 * n * c[m][__builtin_popcount(msk)]);
  }
  printf("%.12f\n", ans);
  return;
}
int main() {
  MEI();
  ZI();
  return 0;
}
