import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		char[] arr = sc.next().toCharArray();
		long length = arr.length;
		HashMap<Character, ArrayList<Integer>> map = new HashMap<>();
		for (int i = 'a'; i <= 'z'; i++) {
		    map.put((char)i, new ArrayList<Integer>());
		}
		for (int i = 0; i < length; i++) {
		    map.get(arr[i]).add(i);
		}
		long sum = 0;
		for (ArrayList<Integer> list : map.values()) {
		    int start = 0;
		    for (int x : list) {
		        long size = x - start;
		        sum += size * (size + 1) / 2;
		        start = x + 1;
		    }
		    long size = length - start;
		    sum += size * (size + 1) / 2;
		}
		System.out.println((26 * length * (length + 1) / 2 - (double)sum) / length / (length + 1) * 2);
  }
}