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1.000000000000000
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#include <bits/stdc++.h>
using namespace std;
int n, ans, cnt[26];
char str[5010];
bool vis[26];
inline int solve(char ch) {
  int ret = 0, tmp;
  for (int k = 1; k < n; ++k) {
    memset(cnt, 0, sizeof cnt);
    tmp = 0;
    for (int i = 0; i < n; ++i)
      if (str[i] == ch) ++cnt[str[(i + k) % n] - 'a'];
    for (int i = 0; i < 26; ++i)
      if (cnt[i] == 1) ++tmp;
    if (tmp > ret) ret = tmp;
  }
  return ret;
}
int main() {
  scanf("%s", str);
  n = strlen(str);
  for (int i = 0; i < n; ++i)
    if (!vis[str[i] - 'a']) {
      vis[str[i] - 'a'] = true;
      ans += solve(str[i]);
    }
  printf("%.8f\n", (double)ans / n);
  return 0;
}
