#include <bits/stdc++.h>
using namespace std;
int main() {
  int l, n;
  cin >> n >> l;
  vector<int> v;
  for (int i = 0; i < n; i++) {
    int a;
    cin >> a;
    v.push_back(a);
  }
  sort(v.begin(), v.end());
  double d = v[0] / 2.0;
  double x, y;
  for (int i = 0; i < n - 1; i++) {
    x = v[i] + d;
    y = v[i + 1] - d;
    if (x < y) {
      d = d + ((y - x) / 2);
    }
  }
  if ((l - v[n - 1]) > d) d = l - v[n - 1];
  if (v[0] > d) d = v[0];
  printf("%.10lf", d);
}
