#include <bits/stdc++.h>
using namespace std;
int n, a;
double b, arr[100000];
int main() {
  cin >> n >> a;
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + n);
  b = max(arr[0], a - arr[n - 1]);
  for (int j = 0; j < n - 1; j++) {
    double ans = (arr[j + 1] - arr[j]) / 2;
    if (ans > b) {
      b = ans;
    }
  }
  cout.setf(ios::fixed);
  cout.precision(10);
  cout << b << endl;
}
