#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
using namespace std;
int main(){
	cout << fixed << setprecision(10);
	string S;
	cin >> S;
	int N = S.size();
	vector<vector<int>> p(26);
	for (int i = 0; i < 26; i++){
		p[i].push_back(-1);
	}
	for (int i = 0; i < N; i++){
		p[S[i] - 'a'].push_back(i);
	}
	for (int i = 0; i < 26; i++){
		p[i].push_back(N);
	}
	double ans = 0;
	for (int i = 0; i < 26; i++){
		long long sum = (long long) N * (N + 1) / 2;
		int M = p[i].size() - 1;
		for (int j = 0; j < M; j++){
			sum -= (long long) (p[i][j + 1] - p[i][j] - 1) * (p[i][j + 1] - p[i][j]) / 2;
		}
		ans += (double) sum / ((long long) N * (N + 1) / 2);
	}
	cout << ans << endl;
}