#include<stdio.h>
#include<stdlib.h>
#include<stdint.h>
#include<inttypes.h>
#include<string.h>

typedef int32_t i32;
typedef int64_t i64;

#define ALLOC(size,type) ((type*)calloc((size),sizeof(type)))

i32 popcnt (i32 v) {
  v = (v & 0x55555555) + ((v >> 1) & 0x55555555);
  v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
  v = (v & 0x0F0F0F0F) + ((v >> 4) & 0x0F0F0F0F);
  v = (v & 0x00FF00FF) + ((v >> 8) & 0x00FF00FF);
  return (v & 0x0000FFFF) + (v >> 16);
}

typedef struct count_val {
  i32 cnt;
  i32 val;
} node;

void run (void) {
  char *s = ALLOC (1000000 + 1, char);
  scanf ("%s", s);
  const i32 n = strlen (s);
  node *p = ALLOC (26, node);
  i32 len = 0;
  i64 sum = 0;
  for (i32 i = 0; i < n; ++i) {
    i32 k = s[i] - 'a';
    for (i32 j = 0; j < len; ++j) {
      p[j].val |= 1 << k;
    }
    p[len++] = (node) {1, 1 << k};
    i32 nlen = 1;
    for (i32 j = 1; j < len; ++j) {
      if (p[j].val == p[nlen - 1].val) {
        p[nlen - 1].cnt += p[j].cnt;
      } else {
        p[nlen++] = p[j];
      }
    }
    len = nlen;
    for (i32 j = 0; j < len; ++j) {
      sum += popcnt (p[j].val) * p[j].cnt;
    }
  }
  double ans = (double) sum / n / (n + 1) * 2;
  printf ("%.6f\n", ans);
}

int main (void) {
  run();
  return 0;
}