#include <bits/stdc++.h> using namespace std; int main(void){ string s; cin >> s; vector<int> lastPos(30); vector<long long> dp(s.length()+1); for (int i = 1; i <= s.length(); i++) { int c = s[i-1] - 'a'; dp[i] = dp[i-1] + i - lastPos[c]; lastPos[c] = i; } long long ans = 0; for (int i = 0; i <= s.length(); i++) { ans += dp[i]; } cout << (double)ans/((s.length()+1)*s.length()/2) << endl; }