#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l, r;
  cin >> n >> l;
  int p[n];
  for (int i = 0; i < n; i++) cin >> p[i];
  sort(p, p + n);
  r = max(p[0], l - p[n - 1]) * 2;
  for (int i = 0; i < n - 1; i++) r = max(r, p[i + 1] - p[i]);
  cout.precision(20);
  cout << fixed << r / 2.0 << endl;
  return 0;
}
