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/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner scan=new Scanner(System.in);
		String s=scan.next();
		int[][] memo=new int[26][5001];
		int[] sz=new int[26];
		for(int i=0;i<26;i++){sz[i]=0;}
		int n=s.length();
		for(int i=0;i<n;i++){
			char c=s.charAt(i);
			int p=c-'a';
			memo[p][sz[p]]=i;
			sz[p]++;
		}
		int res=0;
		for(int i=0;i<26;i++){
			int tot=0;
			for(int d=0;d<=n;d++){
				int[] fr=new int[26];
				for(int j=0;j<26;j++){fr[j]=0;}
				for(int k=0;k<sz[i];k++){
					int ind=(memo[i][k]+d)%n;
					int v=s.charAt(ind)-'a';
					fr[v]++;
				}
				int count=0;
				for(int j=0;j<26;j++){
					if(fr[j]==1){
						count++;
					}
				}
				tot=(int)Math.max(tot,count);
			}
			res+=tot;
		}
		System.out.println(1.0*res/n);
	}
}