#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  long long l;
  cin >> n >> l;
  long long *lanterns = new long long[n];
  for (long long i = 0; i < n; i++) cin >> lanterns[i];
  double maxDistance = 0;
  sort(lanterns, lanterns + n);
  for (long long i = 0; i < n - 1; i++)
    if (lanterns[i + 1] - lanterns[i] > maxDistance)
      maxDistance = lanterns[i + 1] - lanterns[i];
  if (maxDistance < lanterns[0] * 2) maxDistance = lanterns[0] * 2;
  if (maxDistance < (l - lanterns[n - 1]) * 2)
    maxDistance = (l - lanterns[n - 1]) * 2;
  cout << fixed;
  cout << maxDistance / 2;
  return 0;
}
