#include <bits/stdc++.h>
using namespace std;
int ls[1010];
int main() {
  int n, l;
  cin >> n >> l;
  ls[0] = 0;
  n++;
  for (int i = 1; i < n; i++) {
    cin >> ls[i];
  }
  ls[n] = l;
  n++;
  sort(ls, ls + n);
  long long d = max(ls[1] - ls[0], ls[n - 1] - ls[n - 2]);
  d *= 2;
  for (int i = 1; i + 1 < n - 1; i++) {
    int t0, t1;
    t0 = ls[i];
    t1 = ls[i + 1];
    if (t1 - t0 > d) d = t1 - t0;
  }
  double r = d;
  r /= 2;
  cout << setprecision(18) << r << endl;
}
