#include <bits/stdc++.h>
using namespace std;
template <class c>
struct rge {
  c b, e;
};
template <class c>
rge<c> range(c i, c j) {
  return rge<c>{i, j};
}
template <class c>
auto dud(c *x) -> decltype(cerr << *x, 0);
template <class c>
char dud(...);
struct debug {
  template <class c>
  debug &operator<<(const c &) {
    return *this;
  }
};
using ii = pair<int, int>;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using ld = long double;
const int inf = 0x3f3f3f3f;
const ll linf = 0x3f3f3f3f3f3f3f3fll;
template <typename T>
T abs(T x) {
  return x < 0 ? -x : x;
}
template <typename T>
void max_self(T &a, T b) {
  a = max(a, b);
}
template <typename T>
void min_self(T &a, T b) {
  a = min(a, b);
}
const int MAX = 5010;
int dp[MAX][30][30];
int cnt[30];
void solve(int tc) {
  string s;
  cin >> s;
  int n = (int)s.size();
  map<pair<int, pair<char, char> >, int> count;
  for (int i = 0; i < n; ++i) {
    int let = s[i] - 'a';
    cnt[let]++;
    for (int j = 1; j < n; ++j) {
      dp[j][let][s[(i + j) % n] - 'a']++;
    }
  }
  double ans = 0;
  for (int i = 0; i <= 25; ++i) {
    if (cnt[i] == 0) continue;
    double best = 0;
    for (int j = 1; j < n; ++j) {
      int tot = 0;
      int good = 0;
      for (int k = 0; k <= 25; ++k) {
        if (cnt[i] == 0) continue;
        good += dp[j][i][k] == 1;
        tot += dp[j][i][k];
      }
      max_self(best, 1.0 * good / tot);
    }
    ans += best * cnt[i] / n;
  }
  cout << std::fixed << std::setprecision((12)) << (ans);
}
int main() {
  int tc = 1;
  ios::sync_with_stdio(false);
  cin.tie(0);
  for (int i = 1; i <= tc; ++i) {
    solve(i);
  }
  return 0;
}
