#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, length;
  scanf("%d %d", &n, &length);
  int ara[n], d;
  for (int i = 0; i < n; i++) scanf("%d", &ara[i]);
  sort(ara, ara + n);
  d = max(ara[0], length - ara[n - 1]) * 2;
  for (int i = 0; i < n - 1; i++) d = max(d, ara[i + 1] - ara[i]);
  printf("%0.20lf\n", (double)d / 2.0);
  return 0;
}
