#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000 * 1000 * 1000 + 7;
const long long INF = 1ll * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 + 7;
const long long MOD2 = 998244353;
const long long N = 1000 * 100 + 10;
const long long N2 = 70;
const long double PI = 3.141592653589793;
long long gcd(long long a, long long b) {
  if (!b) return a;
  return gcd(b, a % b);
}
long long power(long long x, long long y, long long p = LLONG_MAX) {
  long long res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long lcm(long long a, long long b) { return (a * b) / gcd(a, b); }
void IO() {}
string s;
long long n;
map<char, long long> count1;
long long con(vector<long long> v, long long in) {
  count1.clear();
  for (long long i = 0; i < (long long)v.size(); i++) {
    count1[(s[(v[i] + in) % n])]++;
  }
  long long cnt2 = 0;
  for (auto i : count1) {
    if (i.second == 1) {
      cnt2++;
    }
  }
  return cnt2;
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(20);
  cin >> s;
  n = s.length();
  map<char, vector<long long>> mp;
  for (long long i = 0; i < n; i++) {
    mp[s[i]].push_back(i);
  }
  long long nw = 0;
  for (auto i : mp) {
    if ((i.second).size() == 1) {
      nw++;
      continue;
    }
    vector<long long> pos = i.second;
    long long cnt = pos[1] - pos[0];
    long long m = 0;
    for (long long j = 1; j <= n; j++) {
      m = max(m, con(pos, j));
    }
    nw += m;
  }
  cout << (long double)((long double)nw / (long double)n) << '\n';
  return 0;
}
