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

int main() {
	string S;
	cin >> S;
    int use[26] = {};
    int64_t sum = (int64_t)13 * S.size() * (S.size() + 1);
    for (int i = 0;i < S.size();++ i) for (int j = 0;j < 26;++ j) sum -= use[j] = S[i] == j + 'a' ? 0 : use[j] + 1;
	cout << setprecision(3) << fixed << sum * 2.0 / S.size() / (S.size() + 1) << endl;
	return 0;
}