#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  int k = 0;
  int a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n);
  k = 2 * max(a[0], m - a[n - 1]);
  for (int i = 0; i < n - 1; i++) {
    k = max(k, a[i + 1] - a[i]);
  }
  double kk = k;
  cout << setprecision(10) << kk / 2;
  return 0;
}
