#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;
    
}