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 bababbdaee
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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;
const int N = 1e6;
void nexti(int &i, int length) {
  i++;
  if (i == length) i = 0;
}
vector<int> v[N];
signed main() {
  string s;
  cin >> s;
  int n = s.size();
  for (int i = 0; i < n; i++) v[s[i]].push_back(i);
  double sum = 0;
  for (int i = 'a'; i <= 'z'; i++) {
    double maxs = 0;
    unordered_map<char, int> ma;
    if (v[i].size() == 0) continue;
    for (int j = 0; j < n; j++) {
      for (auto k : v[i]) ma[s[k]]++;
      int mx = 0;
      for (auto k : ma)
        if (k.second == 1) mx++;
      maxs = max(maxs, (double)mx / v[i].size());
      for (int k = 0; k < v[i].size(); k++) nexti(v[i][k], n);
      ma.clear();
    }
    sum += maxs * v[i].size();
  }
  cout.precision(20);
  cout << sum / n << endl;
}
