int main ()
{
	int l,a[1001]={0},s,sum;
	sum=0;
	scanf("%d%d",&l,&s);
		for(int i=1;i<=s;i++)
		{
			int right,left;
			scanf("%d%d",&left,&right);
			for(i=left;i<=right;i++)
				a[i]+=1;
		}
		for(int i=0;i<=l;i++)
		{
			if(a[i]==0)
			sum++;
		}
	 printf("%d",sum);
	 return 0;
}