#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#include<cassert>
#include<cmath>
#include<ctime>
#include<iomanip>
#include<numeric>
#include<stack>
#include<queue>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
#include<bitset>
#include<random>
#include<functional>
#include<utility>
using namespace std;
string S;
vector<int> idx[26];
void solve()
{
	cin >> S;
	int N = (int)S.size();
	for(int i = 0;i < 26;i++) idx[i].push_back(-1);
	for(int i = 0;i < N;i++) idx[S[i]-'a'].push_back(i);
	for(int i = 0;i < 26;i++) idx[i].push_back(N);
	long long num = 0,den = (long long)N*(N+1)/2;
	for(int i = 0;i < 26;i++)
	{
		long long cur = (long long)N*(N+1)/2;
		for(int j = 0;j+1 < (int)idx[i].size();j++)
		{
			int d = idx[i][j+1]-idx[i][j]-1;
			cur -= (long long)d*(d+1)/2;
		}
		num += cur;
	}
	cout << fixed << setprecision(20) << (long double)num/den << "\n";
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt = 1;
	/* cin >> tt; */
	while(tt--) solve();
}