#include <bits/stdc++.h>
using namespace std;
long long read() {
  long long w = 0, f = 1;
  char c = ' ';
  while (c < '0' || c > '9') c = getchar(), f = c == '-' ? -1 : f;
  while (c >= '0' && c <= '9') w = w * 10 + c - 48, c = getchar();
  return w * f;
}
long long n, a[5005], c[30], cnt[30][5005][30], p;
char s[5005];
signed main() {
  scanf("%s", s);
  n = strlen(s);
  for (long long i = 0; i < n; i++) a[i] = s[i] - 'a' + 1;
  for (long long i = 0; i < n; i++) {
    c[a[i]]++;
    for (long long j = 0; j < n; j++) cnt[a[i]][j][a[(i + j) % n]]++;
  }
  for (long long i = 1; i <= 26; i++) {
    long long maxn = 0;
    for (long long j = 0; j < n; j++) {
      long long x = 0;
      for (long long k = 1; k <= 26; k++)
        if (cnt[i][j][k] == 1) x++;
      maxn = max(maxn, x);
    }
    p += maxn;
  }
  printf("%.14lf\n", (double)p / n);
  return 0;
}
