#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main() {
    string s;
    cin >> s;
    int n = (int)s.size();

    int p[256];
    for (int j = 0; j < 256; j++) {
        p[j] = -1;
    }

    double r = 0, t = 0;
    for (int i = 0; i < n; i++) {
        int d = i - p[s[i]];
        p[s[i]] = i;
        t = t + d;
        r += t;
    }

    printf("%.8f\n", r / ((double)n * (n + 1) / 2));

    return 0;
}