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 bababbdaee
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 bbababaaababaabbbbbabbbbbbaaabbabaaaaabbbbbaaaabbbbabaabaabababbbabbabbabaaababbabbababaaaaabaaaabbb
 cadbcdddda
 kpsaloedscghjeaqadfhmlibjepjafdomkkorinrpakondtnrnknbqarbejcenrlsbfgdbsdmkpphbkdnbitjfcofsjibssmmlll
 aaabbbaaaabbbbaaabbbbbaabbbbaaababbaaabbbbaaabbbbababbbbaaabbbbaaabbbbbaabbbbaaabbbbaaabbbb
 ibledofnibedebifmnjdoaijeghajecbkjaebbkofnacceaodiifbhgkihkibddneeiemacodeafeaiiiaoajhmkjffbmmiehebhokfklhbkeoanoajdedjdlkbhenidclagggfhhhldfleccgmjbkhaginlhabkabagikalccndciokabfaebjkndf
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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>
int n, m;
int c[30][10000][30];
char s[10000];
int ans, best;
int main() {
  scanf("%s", s);
  n = strlen(s);
  int a, b;
  for (int i = 0; i < n; ++i)
    for (int j = 1; j < n; ++j) {
      a = s[i] - 'a';
      b = s[(i + j) % n] - 'a';
      ++c[a][j][b];
    }
  ans = 0;
  int temp;
  for (int i = 0; i < 26; ++i) {
    best = 0;
    for (int j = 1; j < n; ++j) {
      temp = 0;
      for (int k = 0; k < 26; ++k)
        if (c[i][j][k] == 1) ++temp;
      if (temp > best) best = temp;
    }
    ans += best;
  }
  printf("%.10lf", double(ans) / n);
  return 0;
}
