#include <bits/stdc++.h>
using namespace std;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

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

    auto ctoi = [](char c) { return int(c - 'a'); };
    auto dp = vec<array<int, 'z' - 'a' + 1>>(uns, s.size() + 1);
    fill(begin(dp[0]), end(dp[0]), 0);

    for (int i = 1; i <= s.size(); ++i) {
        for (char c = 'a'; c <= 'z'; ++c) {
            if (s[i - 1] == c) {
                dp[i][ctoi(c)] = i;
            }
            else {
                dp[i][ctoi(c)] = dp[i - 1][ctoi(c)];
            }
        }
    }

    int64_t ans = 0, cnt = 0;
    for (int i = 1; i <= s.size(); ++i) {
        cnt += i - dp[i - 1][ctoi(s[i - 1])];
        ans += cnt;
    }

    cout << fixed << setprecision(5) << (ans / (double)((s.size() + 1) * s.size() / 2)) << endl;
}