/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.06.16 17:01:13
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    string s;
    cin >> s;
    ll n = s.size();
    double ans = n * (n + 1) *(n + 1) / 2 - n * (n + 1) * (2 * n + 1) / 6;
    vector< int > pre(26, -1);
    for (int i = 0; i < n; i++) {
        if (pre[s[i] - 'a'] != -1) {
            ans -= (n-i)*(pre[s[i]-'a'] + 1);
        }
        pre[s[i] - 'a'] = i;
    }
    printf("%.10f\n",ans*2/n/(n+1));
    return 0;
}