#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
string s;
int n;
long long sum_cnt;
long long calc(int l, int r) {
  return (r - l - 1) * (r - l) / 2;
}
long long cnt(int c) {
  long long res = 0;
  int pre = -1;
  bool in = false;
  for (int i = 0; i < n; i++) {
    if (s[i] != c + 'a') continue;
    in = true;
    res += calc(pre, i);
    pre = i;
  } 
  if (s.back() != c + 'a') res += calc(pre, n);
  if (!in) return 0;
  return sum_cnt - res;
}
signed main() { 
  ios::sync_with_stdio(false); cin.tie(0);
  cin >> s;
  n = (int) s.size();
  sum_cnt = 1LL * n * (n + 1) / 2;
  long long tot = 0;
  cerr << sum_cnt << endl;
  for (int i = 0; i < 26; i++) {
    tot += cnt(i);
    cerr << "i = " << i << " " << cnt(i) << endl;
  }
  cout << setprecision(20) << fixed << 1.0 * tot / sum_cnt << endl;
  return 0;
}