#include <bits/stdc++.h>
using namespace std;
inline long long mod(long long n, long long m) {
  long long ret = n % m;
  if (ret < 0) ret += m;
  return ret;
}
long long gcd(long long a, long long b) {
  return (b == 0LL ? a : gcd(b, a % b));
}
long long exp(long long a, long long b, long long m) {
  if (b == 0LL) return 1LL;
  if (b == 1LL) return mod(a, m);
  long long k = mod(exp(a, b / 2, m), m);
  if (b & 1LL) {
    return mod(a * mod(k * k, m), m);
  } else
    return mod(k * k, m);
}
const long long N = 5050;
vector<string> v[30];
long long cnt[N][30];
long long n;
long long acha(long long id) {
  long long res = 0;
  if (v[id].size() == 0) return res;
  for (long long i = 0; i < N; i++) {
    for (long long j = 0; j < 30; j++) {
      cnt[i][j] = 0;
    }
  }
  for (string t : v[id]) {
    for (long long i = 1; i < n; i++) {
      cnt[i][t[i] - 'a']++;
    }
  }
  for (long long i = 1; i < n; i++) {
    long long aqui = 0;
    for (long long j = 0; j < 26; j++) {
      aqui += (cnt[i][j] == 1);
    }
    res = max(res, aqui);
  }
  return res;
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  string s;
  cin >> s;
  string aux = s + s;
  n = s.size();
  for (long long k = 0; k < n; k++) {
    string t = aux.substr(k, n);
    v[t[0] - 'a'].push_back(t);
  }
  long long tot = n;
  long long good = 0;
  for (long long i = 0; i < 26; i++) good += acha(i);
  double ans = 1.0 * good / tot;
  cout << setprecision(10) << fixed;
  cout << ans << "\n";
}
