#include <bits/stdc++.h>
using namespace std;
double lantern[1005];
int main() {
  int jmllantern;
  double pnjgstreet;
  scanf("%d %lf", &jmllantern, &pnjgstreet);
  for (int i = 0; i < jmllantern; i++) {
    scanf("%lf", &lantern[i]);
  }
  sort(lantern, lantern + jmllantern);
  double maks = 0.0;
  maks = max(maks, lantern[0] - 0.0);
  maks = max(maks, pnjgstreet - lantern[jmllantern - 1]);
  for (int i = 0; i < jmllantern - 1; i++) {
    maks = max(maks, (lantern[i + 1] - lantern[i]) / 2);
  }
  printf("%.10lf", maks);
  return 0;
}
