finding shoes

Problem
Chef has N friends. Chef promised that he would gift a pair of shoes (consisting of one left shoe and one right shoe) to each of his 
N friends. Chef was about to go to the marketplace to buy shoes, but he suddenly remembers that he already had M left shoes.
What is the minimum number of extra shoes that Chef will have to buy to ensure that he is able to gift a pair of shoes to each of his N friends?
For example, if N=2,M=4, then Chef already has 4 left shoes, so he must buy 2 extra right shoes to form 2 pairs of shoes.
Therefore Chef must buy at least 2 extra shoes to ensure that he is able to get N=2 pairs of shoes.


/* package codechef; // 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. */
class Codechef
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc=new Scanner(System.in);
		int t=sc.nextInt();
		for(int i=0;i<t;i++){
		int a=sc.nextInt();
		int b=sc.nextInt();
		if(a<b){
		    System.out.println(a);
		}else
		{
		    System.out.println((a-b)+a);
		} // your code goes here
	}
	}
}