#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  cin >> a >> b;
  double k[a];
  for (int i = 0; i < a; i++) {
    cin >> k[i];
  }
  sort(k, k + a);
  double sm = k[0], big = b - k[a - 1], h = 0;
  for (int i = 1; i < a; i++) {
    h = max(h, (k[i] - k[i - 1]));
  }
  h = h / 2;
  sm = max(sm, big);
  sm = max(sm, h);
  printf("%.9lf", sm);
}
