#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  string S;
  cin >> S;

  ll N = S.size();

  vector<vector<ll>> vs(26, {-1});
  for(ll i = 0; i < N; i++) { vs[S[i] - 'a'].emplace_back(i); }

  long double ans = 0;
  for(auto &v : vs) {
    ll n = v.size();
    for(ll i = 1; i < n; i++) { ans += (v[i] - v[i - 1]) * (N - v[i]); }
  }


  ans /= (N * (N + 1) / 2);
  cout << fixed << setprecision(12) << ans << "\n";
}