#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
vector<vector<long double>> vec(30);
int main() {
	long double ans;
	string s;
	cin >> s;
	long double n = s.size();
	ans = n * (n + 1) / 2 * 26;
	for (int i = 0; i < 26; i++) {
		vec[i].emplace_back(0);
	}
	for (int i = 1; i <= n; i++) {
		vec[int(s[i - 1] - 'a')].emplace_back(i);
	}
	for (int i = 0; i < 26; i++) {
		vec[i].emplace_back(n + 1);
	}
	for (int i = 0; i < 26; i++) {
		for (int j = 1; j < vec[i].size(); j++) {
			long double m = vec[i][j] - vec[i][j - 1] - 1;
			ans -= m * (m + 1) / 2;
		}
	}
	cout << fixed << setprecision(16) << ans / (n * (n + 1) / 2) << endl;
}