/* -*- coding: utf-8 -*-
 *
 * 852.cc:  No.852 連続部分文字列 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 1000000;

/* typedef */

typedef long long ll;

/* global variables */

char s[MAX_N + 4];
int pps[26];

/* subroutines */

/* main */

int main() {
  scanf("%s", s);

  memset(pps, -1, sizeof(pps));
  int n = 0;
  ll sum = 0, d = 0;

  for (; s[n]; n++) {
    int c = s[n] - 'a';
    d += n - pps[c];
    sum += d;
    //printf("%d: %lld %lld %d\n", n, d, sum, pps[n]);
    pps[c] = n;
  }

  ll cnt = (ll)n * (n + 1) / 2;
  printf("%.8lf\n", (double)sum / cnt);
  return 0;
}