#include<stdio.h>
int main(void)
{
	int l,m,u,v,count,i,k;
	scanf("%d %d",&l,&m);
	char trees[**l+1**]={'0'};
	for(i=0;i<=l;i++)
		trees[i]='1';		
	for(k=0;k<m;k++)
	{
		scanf("%d %d",&u,&v);
		for(i=u;i<=v;i++)
			trees[i]='0';
	}
	for(i=0;i<=l;i++)
		if(trees[i]=='1')
			count++;
	printf("%d",count);
	return 0;
}