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 cadbcdddda
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1.000000000000000
 0.000000000000000
 0.450000000000000
 0.960000000000000
 1.000000000000000
 0.800000000000000
 1.000000000000000
 0.080000000000000
 0.840000000000000
 0.000000000000000
 0.800000000000000
 1.000000000000000
 0.000000000000000
 0.786096256684492
 1.000000000000000
 0.438502673796791

#include <bits/stdc++.h>
using namespace std;
int n, m, k;
int dp[5005][26][26], cnt[26];
double t[26];
int main() {
  string s;
  cin >> s;
  n = s.size();
  for (int i = 0; i < n; i++) cnt[s[i] - 'a']++;
  for (int i = 0; i < 26; i++) t[i] = (double)cnt[i] / (double)n;
  double ans = 0;
  for (int i = 0; i < n; i++) {
    for (int len = 1; len < n; len++) {
      dp[len][s[i] - 'a'][s[(i + len) % n] - 'a']++;
    }
  }
  for (int i = 0; i < 26; i++) {
    int res = 0;
    for (int len = 1; len < n; len++) {
      int cur = 0;
      for (int j = 0; j < 26; j++) {
        if (dp[len][i][j] == 1) cur++;
      }
      res = max(res, cur);
    }
    if (cnt[i] != 0) ans += t[i] * (res / (double)cnt[i]);
  }
  cout.precision(8);
  cout << fixed << ans;
  return 0;
}
