#include<iostream>
#include<cstdint>
#include<iomanip>
#include<set>
using namespace std; // 575

int main() {
	string S;
	cin >> S;
	set<int> alpha[26];
	int dp = 0, use[26];
	int64_t sum = 0;
	for (int i = 0;i < 26;++ i) {
		alpha[i].insert(-1);
		alpha[i].insert(S.size());
	}
	for (int i = 0;i < S.size();++ i) alpha[S[i] - 'a'].insert(i);
	for (int i = 0;i < S.size();++ i) sum += (*alpha[S[i] - 'a'].upper_bound(i) - i) * (i + 1);
	cout << setprecision(3) << fixed << sum * 2.0 / S.size() / (S.size() + 1) << endl;
	return 0;
}