import java.io.*;
import java.util.*;
public class Main
{
	public static void main(String[] args) throws Exception
	{
		Scanner cin=new Scanner(System.in);
		long L,M,temp;
		int i;
		while(true)
		{
			L=cin.nextInt();
			M=cin.nextInt();
			if(M==0&&L==0) break;
			for(i=0,temp=L+1;i<M;i++)
			{
				temp+=cin.nextInt();
				temp-=cin.nextInt();
				temp-=1;
			}
			System.out.println(temp);
		}
	}
}