#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

const long long MAX = 5100000;
const long long INF = 1LL << 60;
const long long mod = 1000000007LL;
//const long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;

int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/

	string s; cin >> s;
	long double res = 0;
	vector<vector<char>> alp(26);
	for (ll i = 0; i < s.size(); i++) {
		alp[s[i] - 'a'].push_back(i);
	}

	for (ll i = 0; i < 26; i++) {
		for (ll j = 0; j < (ll)alp[i].size() - 1; j++) {
			res += (alp[i][j + 1] - alp[i][j]) * (alp[i][j] + 1);
		}
		if(alp[i].size() > 0) res += (s.size() - alp[i].back()) * (alp[i].back() + 1);
	}
	res /= (double)(s.size() * (s.size() + 1) / 2);
	cout << fixed << setprecision(12) << res << endl;
	return 0;
}