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

int main() {
  ios::sync_with_stdio(false);

  string S;
  cin >> S;

  vector<int64_t> dp(S.size() + 1);
  vector<int> pre(256, -1);
  for (int i = 0; i < S.size(); ++i) {
    dp[i + 1] = dp[i] + i - pre[S[i]];
    pre[S[i]] = i;
  }
  int64_t sdp = accumulate(dp.begin(), dp.end(), 0LL);

  int64_t n = 1LL * S.size() * (S.size() + 1) / 2;
  double ans = 1.0 * sdp / n;
  cout << fixed << setprecision(9) << ans << endl;

  return 0;
}