#include <stdio.h>
#include <stdlib.h>
//#include <string.h>
int main(void)
{
	int L,M;
	int a,b;
	int *p;
	int i;
	int count=0;
	while((scanf("%d%d",&L,&M),L))
	{
		p=(int *)malloc(sizeof(int)*(L+1));
		//memset(p,0,L+1);
		for(i=0;i<L+1;i++)
		{
			p[i] = 0;
		}
		while(M)
		{
			scanf("%d%d",&a,&b);
			for(i=a;i<=b;i++)
			{
				p[i] = 1;
			}
			M--;
		}
		count = 0;
		for(i=0;i<L+1;i++)
		{
			if(p[i] == 0)
			{
				count++;
			}
		}
		free(p);
		printf("%d\n",count);
	}
	return 0;
}
/**************************************************************
	Problem: 2189
	User: admin
	Language: C
	Result: Accepted
	Time:12 ms
	Memory:1144 kb
****************************************************************/