import java.io.*;
import java.util.*;

public class Main
{
	public static void main(String args[]) throws Exception
	{
		Scanner cin=new Scanner(System.in);
		int money[][]=new int[10000][4];
		int i=0,n=0;
		while(true)
		{
			money[i][0]=cin.nextInt();
			money[i][1]=cin.nextInt();
			money[i][2]=cin.nextInt();
			money[i][3]=cin.nextInt();
			if(money[i][0]==0&&money[i][1]==0&&money[i][2]==0&&money[i][3]==0) break;
			i++;
			n=i;
		}
		for(i=0;i<n;i++)
		{
			System.out.println((money[i][0]-money[i][1]+money[i][2]));
		}
	}
}