using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		N = S.Length;
		int[] ll = new int[27];
		var dl = new Dictionary<char, int>[27];
		for(int i=0;i<27;i++) dl[i] = new Dictionary<char,int>();
		
		double cnt = 0;
		for(int i=0;i<N;i++){
			
			for(int len=0;len<=26;len++){
				if(ll[len] < i) ll[len] = i;
				while(ll[len] < N){
					if(dl[len].ContainsKey(S[ll[len]])){
						dl[len][S[ll[len]]]++;
						ll[len]++;
						continue;
					}
					if(dl[len].Count < len){
						dl[len].Add(S[ll[len]], 1);
						ll[len]++;
						continue;
					}
					break;
				}
			}
			for(int len = 1; len <= 26; len++){
				if(dl[len].Count == len){
					cnt += (double)len * (double)(ll[len] - ll[len - 1]);
				}
			}
			for(int len = 0; len <= 26; len++){
				if(dl[len].ContainsKey(S[i])){
					dl[len][S[i]]--;
					if(dl[len][S[i]] == 0) dl[len].Remove(S[i]);
				}
			}
				
		}
		double tot = (double) N * (double) (N + 1) / 2.0;
		
		Console.WriteLine(cnt / tot);
	}
	
	
	int N;
	String S;
	public Sol(){
		S = rs();
		
	}

	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);}
	static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));}
	static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));}
	static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));}
}