#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, t;
  double j = 0;
  cin >> n >> t;
  double arr[n];
  for (long long i = 0; i < n; i++) cin >> arr[i];
  sort(arr, arr + n);
  for (int i = 0; i < n - 1; i++) {
    if ((arr[i + 1] - arr[i]) / 2 > j) {
      j = (arr[i + 1] - arr[i]) / 2;
    }
  }
  if (arr[0] != 0) {
    if (arr[0] > j) j = arr[0];
  }
  if (arr[n - 1] != t) {
    if (t - arr[n - 1] > j) j = t - arr[n - 1];
  }
  printf("%.10f", j);
  return 0;
}
