#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;
}

ll calc2(string& s) {
    int n = s.size();
    vector<int> nex(26, -1);
    vector<ll> dp(n, 0);
    for (int i = n - 1; i >= 0; i--) {
        for (int j = 0; j < 26; j++) {
            if (nex[j] == -1) continue;
            dp[i] += dp[nex[j]];
        }
        int& tar = nex[s[i] - 'a'];
        if (tar == -1) dp[i]++;
        tar = i;
    }

    ll res = 0;
    for (int i = 0; i < 26; i++) {
        if (nex[i] != -1) res += dp[nex[i]];
    }
    return res;
}

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

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

    ll val2 = n * (n + 1) / 2;//calc2(s);
    //cerr << val2 << endl;
    cout << fixed << setprecision(15) << (long double)val1 / val2 << endl;
    return 0;
}