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 bbababaaababaabbbbbabbbbbbaaabbabaaaaabbbbbaaaabbbbabaabaabababbbabbabbabaaababbabbababaaaaabaaaabbb
 cadbcdddda
 kpsaloedscghjeaqadfhmlibjepjafdomkkorinrpakondtnrnknbqarbejcenrlsbfgdbsdmkpphbkdnbitjfcofsjibssmmlll
 aaabbbaaaabbbbaaabbbbbaabbbbaaababbaaabbbbaaabbbbababbbbaaabbbbaaabbbbbaabbbbaaabbbbaaabbbb
 ibledofnibedebifmnjdoaijeghajecbkjaebbkofnacceaodiifbhgkihkibddneeiemacodeafeaiiiaoajhmkjffbmmiehebhokfklhbkeoanoajdedjdlkbhenidclagggfhhhldfleccgmjbkhaginlhabkabagikalccndciokabfaebjkndf
 abbacba
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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;
int n;
int hs(char a, char b) { return (a - 'a') * 27 + (b - 'a'); }
int h[30 * 30][3050];
int ans[30][3050];
int main() {
  cin >> s;
  n = s.size();
  for (int i = 0; i < n; i++) {
    for (int j = 1; j < n; j++) {
      char c1 = s[i], c2 = s[(i + j) % n];
      int x = hs(c1, c2);
      h[x][j]++;
    }
  }
  for (int i = 0; i < n; i++) {
    for (int j = 1; j < n; j++) {
      char c1 = s[i], c2 = s[(i + j) % n];
      int x = hs(c1, c2);
      if (h[x][j] == 1) ans[s[i] - 'a'][j]++;
    }
  }
  int cnt = 0;
  for (int i = 0; i < 26; i++) {
    int t = 0;
    for (int j = 0; j < n; j++) t = max(t, ans[i][j]);
    cnt += t;
  }
  printf("%.9lf", cnt * 1.0 / n);
  return 0;
}
