#include <bits/stdc++.h>
using namespace std;
bool mark[1000];
int main() {
  long long n, i, l;
  double rad = 0, m;
  cin >> n >> l;
  long long lamp[n + 10];
  for (i = 0; i < n; i++) {
    scanf("%lld", &lamp[i]);
  }
  sort(lamp, lamp + n);
  rad = max(lamp[0] - 0, l - lamp[n - 1]);
  i = 0;
  if (lamp[0] == 0) i = 1;
  for (; i < n; i++) {
    m = (lamp[i] - lamp[i - 1]);
    m = m / 2;
    rad = max(rad, m);
  }
  printf("%.12lf\n", rad);
}
