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/* package codechef; // 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 Codechef
{
    long MOD = 1000000007L;
    int INF = 1000000000;
    
    public class Obj implements Comparable<Obj>{
        public int start;
        public int end;
        
        public Obj(int start_, int end_){
            this.start = start_;
            this.end = end_;

        }
        
        public int compareTo(Obj other){
            if (other.start == this.start){
                return this.end - other.end;
            }
            return this.start - other.start;
        }

    }
    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
        Codechef cf = new Codechef();
        cf.Solve();
	}
	
	public void Solve(){
        MyScanner sc = new MyScanner();
        String s = sc.nextLine();
        char[] arr = s.toCharArray();
        int len = arr.length;
        int[] cnt = new int[26];
        boolean[] ok = new boolean[26];
        int[] pos = new int[len];
        int[][] cnt2 = new int[5000][26];
        int num = 0;
        double res = 0.0;
        for (int i = 0; i < 26; ++i){
            char c = (char)(i + 'a');
            int sz = 0;            
            for (int j = 0; j < len; ++j){
                if (c == arr[j]){
                    cnt[i]++;
                    pos[sz++] = j;
                }
            }
            if (sz == 0) continue;
            for (int k = 1; k < len; ++k){
                for (int j = 0; j < 26; ++j){
                    cnt2[k][j] = 0;
                }
            }
            for (int k = 1; k < len; ++k){
                for (int j = 0; j < sz; ++j){
                    int curP = (pos[j] + k) % len;
                    int idx = (int)(arr[curP] - 'a');
                    cnt2[k][idx]++;
                }
            }
            int best = 0;
            for (int k = 1; k < len; ++k){
                int cur = 0;
                for (int j = 0; j < sz; ++j){
                    int curP = (pos[j] + k) % len;
                    int idx = (int)(arr[curP] - 'a');
                    if (cnt2[k][idx] == 1){
                        cur++;
                    }
                }
                best = Math.max(best, cur);
            }
            num += best;
        }
        res = 1.0 * num / len;
        System.out.println(res);
	}
	
	//-----------PrintWriter for faster output---------------------------------
   public static PrintWriter out;
      
   //-----------MyScanner class for faster input----------
   public static class MyScanner {
      BufferedReader br;
      StringTokenizer st;
 
      public MyScanner() {
         br = new BufferedReader(new InputStreamReader(System.in));
      }
 
      String next() {
          while (st == null || !st.hasMoreElements()) {
              try {
                  st = new StringTokenizer(br.readLine());
              } catch (IOException e) {
                  e.printStackTrace();
              }
          }
          return st.nextToken();
      }
 
      int nextInt() {
          return Integer.parseInt(next());
      }
 
      long nextLong() {
          return Long.parseLong(next());
      }
 
      double nextDouble() {
          return Double.parseDouble(next());
      }
 
      String nextLine(){
          String str = "";
      try {
         str = br.readLine();
      } catch (IOException e) {
         e.printStackTrace();
      }
      return str;
      }

   }
}
