#include <bits/stdc++.h>
using namespace std;
vector<int> v[181];
string s[5051], s1;
long double p, q, kol, ans;
int p1[181][181], n, i, j, z, z1;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> s1;
  n = s1.size();
  for (i = 1; i <= n; i++) {
    s1 += s1[0];
    s1.erase(0, 1);
    s[i] = s1;
  }
  for (i = 1; i <= n; i++) v[s[i][0] - 96].push_back(i);
  for (z = 1; z <= 26; z++)
    if (v[z].size() > 0) {
      p = 0.;
      for (z1 = 1; z1 < n; z1++) {
        kol = 0.;
        for (i = 1; i <= 26; i++)
          for (j = 1; j <= 26; j++) p1[i][j] = 0;
        for (i = 0; i < v[z].size(); i++) p1[z][s[v[z][i]][z1] - 96]++;
        for (i = 1; i <= 26; i++)
          for (j = 1; j <= 26; j++)
            if (p1[i][j] == 1) kol++;
        p = max(p, kol);
      }
      q = v[z].size();
      ans += p;
    }
  q = n;
  ans /= q;
  cout << setprecision(10) << fixed;
  cout << ans << '\n';
  return 0;
}
