#include <string>
#include <vector>
#include <iostream>
using namespace std;
int main() {
	cin.tie(0);
	ios_base::sync_with_stdio(false);
	string s;
	cin >> s;
	long long sum = 0;
	for (int i = 1; i <= 26; ++i) {
		vector<int> c(26);
		int cur = 0, ptr = 0;
		for (int j = 0; j < s.size(); ++j) {
			while (ptr != s.size() && cur < i) {
				if (c[s[ptr++] - 'a']++ == 0) ++cur;
			}
			if (cur == i) sum += s.size() - ptr + 1;
			if (--c[s[j] - 'a'] == 0) --cur;
		}
	}
	cout.precision(15);
	cout << fixed << sum / double(1LL * s.size() * (s.size() + 1) / 2) << endl;
	return 0;
}