#include <iostream>
#include <string>
#include <vector>
#include <iomanip>

using namespace std;

int main() {
    string s;
    cin >> s;

    vector<long double> last(26);
    long double ret = 0;
    for (double i = 1; i <= s.size(); i++) {
        last[s[i - 1] - 'a'] = i;
        for (int j = 0; j < 26; j++) {
            ret += last[j];
        }
    }

    long long l = s.size();
    l = l * (l + 1) / 2;
    cout << fixed << setprecision(18) <<ret / l << endl;
    return 0;
}