#include <bits/stdc++.h>
using namespace std;
string s;
long long int n;
double arr[26];
int main() {
  cin >> s;
  memset(arr, 0, sizeof(arr));
  n = s.size();
  double prob = 0;
  vector<vector<int>> v;
  v.resize(26);
  for (int i = 0; i < n; i++) {
    v[s[i] - 97].push_back(i);
  }
  for (int c = 0; c < 26; c++) {
    if (v[c].size() == 0) continue;
    for (int d = 0; d < n; d++) {
      unordered_set<int> s1, s2;
      for (int i = 0; i < v[c].size(); i++) {
        int hsh = 200 * s[v[c][i]] + s[(v[c][i] + d) % n];
        if (s1.count(hsh))
          s2.insert(hsh);
        else
          s1.insert(hsh);
      }
      int a = s1.size() - s2.size();
      arr[c] = max(arr[c], 1. * a);
    }
  }
  for (double d : arr) prob += d;
  prob /= (1. * n);
  cout << setprecision(20) << prob << endl;
}
