import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(scanner.hasNext()){
			int l=scanner.nextInt();
			if(l==0){
				continue;
			}
			int m=scanner.nextInt();
			boolean[] tree=new boolean[10010];
			while(m--!=0){
				int begin=scanner.nextInt();
				int end=scanner.nextInt();
				for (int i = begin; i <= end; i++) {
					tree[i]=true;
				}
			}
			int ans =0;
			for (int i = 0; i <= l; i++) {
				if(tree[i]==false){
					++ans;
				}
			}
			System.out.println(ans);
		}
		
	}

}

/**************************************************************
	Problem: 2189
	User: admin
	Language: Java
	Result: Accepted
	Time:742 ms
	Memory:42440 kb
****************************************************************/