#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
signed main() { 
  ios::sync_with_stdio(false); cin.tie(0);
  string s;
  cin >> s;
  vector<int> pre(26, -1);
  long long tot = 0;
  int n = (int) s.size();
  for (int i = 0; i < n; i++) {
    tot += 1LL * (i - pre[s[i] - 'a']) * (n - i);
    pre[s[i] - 'a'] = i;
  }
  cout << setprecision(20) << fixed << 1.0 * tot / (n * (n + 1) / 2) << endl;
  return 0;
}