#include<bits/stdc++.h>

using namespace std;
using ll = long long;


int main()
{
	string s; ll n;
	cin >> s; n = s.size();
	vector<vector<ll>> pos(26);
	for(auto &i: pos)i.push_back(-1);

	for(ll i = 0; i < n; i++) {
		pos[s[i] - 'a'].push_back(i);
	}
	for(auto &i: pos)i.push_back(n);
	ll unit = n * (n + 1) / 2;

	ll sum = 0;
	for(ll i = 0; i < 26; i++) {
		ll cur = unit;
		ll m = pos[i].size();
		for(ll j = 0; j < m - 1; j++) {
			ll num = pos[i][j+1] - pos[i][j];
			cur -= num * (num - 1) / 2;
		}
		sum += cur;
	}
	cout << fixed <<setprecision(12) << (double)sum / ((n) * (n+1) / 2) << endl; 
}