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#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
const int INF = 0x3f3f3f3f;
const double eps = 1e-8;
const long long mod = 10007;
const int N = 100005;
char str[5005];
int len;
map<char, vector<int> > maps;
void solve() {
  scanf("%s", str);
  len = strlen(str);
  int ans = 0;
  for (int i = 1; i < len; i++) {
    map<pair<char, char>, int> cnt;
    for (int j = 0; j < len; j++) {
      pair<char, char> now = make_pair(str[j], str[(j + i) % len]);
      cnt[now]++;
    }
    int nowans = 0;
    for (auto it = cnt.begin(); it != cnt.end(); it++) {
      if (it->second == 1) nowans++;
    }
    ans = max(ans, nowans);
  }
  printf("%.10f\n", (double)ans / len);
  return;
}
int main() {
  solve();
  return 0;
}
