#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double m;
  cin >> n >> m;
  cout << fixed << showpoint << setprecision(16);
  double a[n + 10];
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  if (n == 1) {
    cout << max(m - a[1], a[1]) << endl;
    return 0;
  }
  sort(a + 1, a + n + 1);
  a[n + 1] = m;
  double ans = 0.0;
  for (int i = 1; i <= n; i++) {
    if (i == 1 && i != n) {
      ans = max(ans, max(a[i], (double)((a[i + 1] - a[i]) / 2.0)));
    } else if (i == n) {
      ans = max(ans, max((a[i + 1] - a[i]), (double)((a[i] - a[i - 1]) / 2.0)));
    } else {
      ans = max(ans, max((double)((a[i + 1] - a[i]) / 2.0),
                         (double)((a[i] - a[i - 1]) / 2.0)));
    }
  }
  cout << ans << endl;
}
