import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    long[] use = new long[26];
    for(int i = 0; i < 26; i++) {
      use[i] = -1;
    }
    long[] dis = new long[s.length()];
    for(int i = 0; i < s.length(); i++) {
      int t = s.charAt(i) - 97;
      dis[i] = i - use[t];
      use[t] = i;
    }
    long[] dp = new long[s.length()];
    dp[0] = 1;
    for(int i = 1; i < s.length(); i++) {
      dp[i] = dp[i - 1] + dis[i];
    }
    long sa = s.length();
    long a = (sa * (sa - 1)) / 2 + sa;
    double ans = (double)dp[s.length() - 1] / (double)a;
    System.out.println(ans);
  }
}