#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    const ll chars = 'z' - 'a' + 1;
    string s;
    cin >> s;
    map<ll, vector<ll>> idx;
    rep(i, len(s)) idx[s[i] - 'a'].push_back(i);
    rep(i, chars) idx[i].push_back(len(s));
    ll cnt = 0;
    rep(i, chars) {
        rep(j, sz(idx[i]) - 1) {
            ll l = idx[i][j];
            ll r = idx[i][j + 1] - idx[i][j] - 1;
            cnt += (l + 1) * (r + 1);
        }
    }
    double ans = (double)cnt / ((1 + len(s)) * len(s) / 2);
    printf("%.10f\n", ans);
    return 0;
}