#include <bits/stdc++.h>
using namespace std;
long long dp[26][26][5000], kol[26];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  long long i, j, t, q, x, p, z, l, r, y, w, k, n, v, f, x1, y1, d;
  string s;
  cin >> s;
  n = s.size();
  for (i = 0; i < n; i++) {
    x = s[i] - 'a';
    kol[x]++;
    for (j = 1; j < n; j++) {
      z = (i + j) % n;
      y = s[z] - 'a';
      dp[x][y][j]++;
    }
  }
  long double p1, p2, maxi, mini, sum;
  sum = 0;
  for (i = 0; i < 26; i++) {
    maxi = 0;
    p1 = kol[i];
    if (p1 != 0) {
      for (d = 1; d < n; d++) {
        mini = 0;
        for (j = 0; j < 26; j++) {
          if (dp[i][j][d] == 1) {
            p2 = 1. / p1;
            mini += p2;
          }
        }
        maxi = max(maxi, mini);
      }
      p1 = n;
      p2 = kol[i];
      p1 = p2 / p1 * maxi;
      sum += p1;
    }
  }
  cout << fixed << setprecision(7) << sum << endl;
  return 0;
}
