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

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