#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l;
  scanf("%d%d", &n, &l);
  vector<int> lant(n);
  for (int i = 0; i < n; i++) scanf("%d", &lant[i]);
  sort(lant.begin(), lant.end());
  int d = 2 * max(lant[0] - 0, l - lant[n - 1]);
  for (int i = 0; i < n - 1; i++) d = max(d, lant[i + 1] - lant[i]);
  printf("%f\n", (double)d / 2);
}
