/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);
		int t = sc.nextInt();
		while(t-- >0){
			int n = sc.nextInt();
			int k = sc.nextInt();
			int [] a = new int[n];
			int l=0;
			int ans=0;
			for(int i=0;i<n;i++){
				int curr = sc.nextInt();
			    ans = Math.max(ans,curr-l);	
			    l = curr;
			}
			ans = Math.max(ans,2*(k-l));
			System.out.println(ans);
 
		}
 
	}
}

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MwozIDcKMSAyIDUKMyA2CjEgMiA1CjEgMTAKNw==

3
3 7
1 2 5
3 6
1 2 5
1 10
7