import java.util.Scanner;
import java.io.*;
import java.util.*;
import java.util.ArrayList;

public class P492B{
    public static void main(String[] args){
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();
        int l = s.nextInt();
        ArrayList<Integer> x = new ArrayList<Integer>();
        for(int i = 0; i<n; i++){
            x.add(s.nextInt());
        }
        x.add(0);
        x.add(l);
        Collections.sort(x);
        double d = 0;
        double lo = 0;
        double hi = l-1;
        while(lo <= hi){
            double mid = (lo+hi)/2;
            boolean b = true;
            for(int i = 0; i<x.size()-1; i++){
                if(x.get(i+1) - x.get(i) > 2*mid){
                    b = false;
                }
            }
            if(b){
                hi = mid - 1;
                d = mid;
            }
            if(!b){
                lo = mid + 1;
            }
        }
        System.out.println(d);
    }
}