#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n, l, t;
  cin >> n >> l;
  vector<int> lant;
  for (int i = 0; i < n; i++) {
    cin >> t;
    lant.push_back(t);
  }
  sort(lant.begin(), lant.end());
  int ma = 0;
  for (int i = 0; i < n - 1; i++) ma = max(ma, (lant[i + 1] - lant[i]));
  ma = max(ma, lant[0] * 2);
  ma = max(ma, (l - lant[n - 1]) * 2);
  printf("%.10f\n", (float)ma / 2);
  return (0);
}
