#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double s[1000], m, a, b, c, max;
  while (cin >> n >> m) {
    for (int i = 0; i < n; i++) {
      cin >> s[i];
    }
    sort(s, s + n);
    a = s[0];
    b = m - s[n - 1];
    if (a > b)
      max = a;
    else
      max = b;
    for (int i = 0; i < n - 1; i++) {
      c = (s[i + 1] - s[i]) / 2;
      if (max < c) {
        max = c;
      }
    }
    printf("%.10lf\n", max);
  }
  return 0;
}
