import java.io.*;
import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        char[] inputs = sc.next().toCharArray();
        ArrayList<TreeSet<Integer>> places = new ArrayList<>();
        for (int i = 0; i < 26; i++) {
            places.add(new TreeSet<>());
            places.get(i).add(inputs.length + 1);
        }
        for (int i = 0; i < inputs.length; i++) {
            places.get(inputs[i] - 'a').add(i + 1);
        }
        double all = (double)inputs.length * (inputs.length + 1) / 2;
        double total = all * 26;
        for (TreeSet<Integer> org : places) {
            int prev = 0;
            for (int x : org) {
                total -= (x - prev) * (x - prev - 1.0) / 2;
                prev = x;
            }
        }
        System.out.println(new java.math.BigDecimal(total / all).toPlainString());
    }
}
class Utilities {
    static String arrayToLineString(Object[] arr) {
        return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n"));
    }
    
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}