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1.000000000000000
 0.000000000000000
 0.450000000000000
 0.960000000000000
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#include <bits/stdc++.h>
inline long long read() {
  long long num = 0, f = 1;
  char c = getchar();
  while (c < 48 || c > 57) {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c > 47 && c < 58) num = num * 10 + (c ^ 48), c = getchar();
  return num * f;
}
char a[5005];
long long aa[26][26][5005];
signed main() {
  scanf("%s", a);
  long long n = strlen(a);
  for (long long i = 0; i < n; i++) a[i] -= 97;
  for (long long i = 0; i < n; i++) {
    for (long long j = 0; j < n; j++) {
      aa[a[i]][a[(i + j) % n]][j]++;
    }
  }
  long long ans = 0;
  for (long long i = 0; i < 26; i++) {
    long long wew = 0, pop = 0;
    for (long long j = 0; j < n; j++) {
      wew = 0;
      for (long long k = 0; k < 26; k++)
        if (aa[i][k][j] == 1) wew++;
      pop = pop > wew ? pop : wew;
    }
    ans += pop;
  }
  printf("%.7lf\n", 1.0 * ans / n);
}
