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 bbababaaababaabbbbbabbbbbbaaabbabaaaaabbbbbaaaabbbbabaabaabababbbabbabbabaaababbabbababaaaaabaaaabbb
 cadbcdddda
 kpsaloedscghjeaqadfhmlibjepjafdomkkorinrpakondtnrnknbqarbejcenrlsbfgdbsdmkpphbkdnbitjfcofsjibssmmlll
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1.000000000000000
 0.000000000000000
 0.450000000000000
 0.960000000000000
 1.000000000000000
 0.800000000000000
 1.000000000000000
 0.080000000000000
 0.840000000000000
 0.000000000000000
 0.800000000000000
 1.000000000000000
 0.000000000000000
 0.786096256684492
 1.000000000000000
 0.438502673796791

#include <bits/stdc++.h>
using namespace std;
string s;
vector<int> cnt[26];
int a[26];
int tag[26];
int main() {
  long long i, j, k, n, m, p;
  double ans;
  while (cin >> s) {
    n = s.size();
    memset(cnt, 0, sizeof(cnt));
    for (i = 0; i < n; i++) cnt[int(s[i]) - 97].push_back(i);
    ans = 0.0;
    for (i = 0; i < 26; i++)
      if (cnt[i].size() > 0) {
        p = 0;
        for (int len = 1; len < n; len++) {
          m = 0;
          memset(tag, 0, sizeof(tag));
          for (j = 0; j < cnt[i].size(); j++) {
            int location = cnt[i][j];
            int check_c = int(s[(location + len) % n]) - 97;
            tag[check_c]++;
          }
          for (j = 0; j < 26; j++)
            if (tag[j] == 1) m++;
          p = max(m, p);
        }
        ans = ans + (double(cnt[i].size()) / n) * double(p) / cnt[i].size();
      }
    cout << fixed << setprecision(10) << ans << endl;
  }
}
