#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;
typedef string::const_iterator State;
#define Ma_PI 3.141592653589793
#define eps 0.00000001
#define LONG_INF 1e18
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define MOD 998244353
#define seg_size 262144
#define REP(a,b) for(long long a = 0;a < b;++a)
vector<complex<double>>  DFT(vector<complex<double>> a) {
	int n = a.size();
	if (n == 1) return a;
	vector<complex<double>> a0(n / 2), a1(n / 2);
	REP(i, n) {
		if (i % 2 == 0) {
			a0[i / 2] = a[i];
		}
		else {
			a1[i / 2] = a[i];
		}
	}
	vector<complex<double>> inversed_a0 = DFT(a0), inversed_a1 = DFT(a1);
	vector<complex<double>> inversed_a(n);
	for (int i = 0; i < n; ++i) {
		complex<double> zeta = complex<double>(cos(2 * Ma_PI * i / n), sin(2 * Ma_PI * i / n));
		inversed_a[i] = inversed_a0[i % (n / 2)] + zeta * inversed_a1[i % (n / 2)];
	}
	return inversed_a;
}
vector<complex<double>> IDFT(vector<complex<double>> inversed_a) {
	int n = inversed_a.size();
	vector<complex<double>> now = DFT(inversed_a);
	reverse(now.begin(), now.end());
	for (int q = now.size() - 1; q >= 1; --q) {
		swap(now[q], now[q - 1]);
	}
	REP(i, n) {
		now[i] /= complex<double>(n, 0);
	}
	return now;
}
vector<complex<double>> conv(vector<complex<double>> a, vector<complex<double>> b) {
	int deg = a.size() + b.size() - 1;
	int n = 1;
	while (n < deg) n <<= 1;
	a.resize(n);
	b.resize(n);
	vector<complex<double>> inversed_a = DFT(a), inversed_b = DFT(b);
	vector<complex<double>> inversed_c(n);
	REP(i, n) {
		inversed_c[i] = inversed_a[i] * inversed_b[i];
	}
	return IDFT(inversed_c);
}
long long inv(long long now) {
	long long hoge = MOD - 2LL;
	long long ans = 1;
	while (hoge != 0) {
		if (hoge % 2 == 1) {
			ans *= now;
			ans %= MOD;
		}
		hoge /= 2;
		now *= now;
		now %= MOD;
	}
	return ans;
}
int main() {
	string s;
	cin >> s;
	long long tmp = s.length();
	long long bunsi = 0;
	long long bunbo = tmp * (tmp + 1LL) / 2LL;
	for (int i = 'a'; i <= 'z'; ++i) {
		long long chose = tmp * (tmp + 1LL) / 2LL;
		long long now = 0;
		for (int q = 0; q < s.length(); ++q) {
			if (s[q] == i) {
				chose -= now * (now + 1LL) / 2LL;
				now = 0;
			}
			else {
				now++;
			}
		}
		chose -= now * (now + 1LL) / 2LL;
		bunsi += chose;
	}
	cout << fixed << setprecision(20);
	cout << (long double)((long double)(bunsi) / (long double)(bunbo)) << endl;
	return 0;
}