import java.util.*;

public class Main {
	public static void main (String[] args) {
    	Scanner sc = new Scanner(System.in);
    	char[] arr = sc.next().toCharArray();
    	int length = arr.length;
    	HashMap<Integer, ArrayList<Integer>> charPlace = new HashMap<>();
    	for (int i = 0; i < 26; i++) {
    	    charPlace.put(i, new ArrayList<>());
    	}
    	for (int i = 0; i < length; i++) {
    	    charPlace.get(arr[i] - 'a').add(i);
    	}
    	int[] idxes = new int[26];
    	int[] places = new int[26];
    	for (int i = 0; i < 26; i++) {
    	    charPlace.get(i).add(length);
    	    places[i] = charPlace.get(i).get(0);
    	}
    	double total = 0;
    	for (int i = 0; i < length; i++) {
    	    for (int j = 0; j < 26; j++) {
    	        total += length - places[j];
    	    }
    	    int idx = arr[i] - 'a';
    	    idxes[idx]++;
    	    places[idx] = charPlace.get(idx).get(idxes[idx]);
    	}
    	double ans = total / length / (length + 1) * 2;
    	System.out.println(ans);
    }
}