#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    string s; cin >> s;
    ll n = s.size();
    ll sum = 0;
    for(char c = 'a'; c <= 'z'; c++) {
        vector<ll> id;
        id.push_back(-1);
        rep(i,n) if(s[i] == c) id.push_back(i);
        id.push_back(n);
        sum += n * (n + 1) / 2;
        rep(i,int(id.size())-1) {
            ll d = id[i + 1] - id[i] - 1;
            sum -= d * (d + 1) / 2;
        }
    }

    cout << fixed << setprecision(20) << (long double)(sum) / (n * (n + 1) / 2) << endl;
}