#include <vector>
#include <iostream>
using namespace std;
int main() {
    string s; cin >> s;
    vector<int> pos[26];
    for (int i = 0; i < s.size(); i++) pos[s[i]-'a'].push_back(i);
    long long ans = 0;
    const long long all = (long long)s.size() * (s.size()+1) / 2;
    for (auto v : pos) {
        if (v.empty()) continue;
        long long sum = 0;
        int t = 0;
        for (int i = 0; i < v.size(); i++) {
            long long u = v[i] - t;
            sum += u * (u + 1) / 2;
            t = v[i] + 1;
        }
        long long u = s.size() - v.back() - 1;
        sum += u * (u + 1) / 2;
        ans += all - sum;
    }
    cout << (double)ans/all << endl;
}