#include <bits/stdc++.h>

using namespace std;

using ll = long long;

ll calc(string& s, char c) {
    ll n = s.size();
    ll res = n * (n + 1) / 2;

    ll cur = 0;
    for (ll i = 0; i < n; i++) {
        if (s[i] == c) {
            ll d = i - cur;
            res -= d * (d + 1) / 2;
            cur = i + 1;
        }
    }

    ll d = n - cur;
    res -= d * (d + 1) / 2;
    return res;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    string s;
    cin >> s;
    int n = s.size();

    ll val1 = 0;
    for (int i = 0; i < 26; i++) {
        val1 += calc(s, 'a' + i);
    }

    ll val2 = n * (n + 1) / 2;
    cout << fixed << setprecision(15) << (long double)val1 / val2 << endl;
    return 0;
}