#include <stdio.h>
int l,m;
int run()
{
	int a[11111],i,j,k,h,t=0;
	for(i=0;i<=l;i++)
		a[i]=1;
	for(i=1;i<=m;i++)
	{
		scanf("%d%d",&j,&k);
		for(h=j;h<=k;h++)
			a[h]=0;
	}
	for(i=0;i<=l;i++)
		t+=a[i];
	return t;
}
int main()
{
	scanf("%d%d",&l,&m);
	while(l!=0)
	{
		printf("%d\n",run());
		l=0;
		scanf("%d%d",&l,&m);
	}
	return 0;
}
/**************************************************************
	Problem: 2189
	User: admin
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:1144 kb
****************************************************************/