#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>
#include<set>
#include<map>

using namespace std;

#define int long long
#define endl "\n"

const long long INF = (long long)1e18;
const long long MOD = (long long)1e9 + 7; 

string yn(bool f){return f?"Yes":"No";}
string YN(bool f){return f?"YES":"NO";}

const int NUM = 26;

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(10);
	
	long double ans = 0;
	vector<int> used;
	int dp = 0, sum = 0;
	string s;
	
	cin>>s;
	
	used.resize(NUM,-1);
	
	for(int i = 0; i < s.size(); i++){
		dp += i - used[s[i] - 'a'];
		sum += dp;
		used[s[i] - 'a'] = i;
	}
	// cout<<sum<<endl;
	cout<<((double)sum/((int)s.size()*(s.size()+1)/2))<<endl;
	
	return 0;
}