#include<stdio.h>
int main()
{
	
	int length, m, i, j, u, v, number=0;
	
	int a[1000] ;
	
	scanf("%d%d", &length, &m);
	 
	for(i = 0; i <= length; i++) {
		
		a[i] = i;
	}

	for(i = 0; i < m; i ++) {
		
		scanf("%d%d",&u,&v);
		
		for(j = 0; j <= length; j ++) {
			
			if(a[j] >= u && a[j] <= v) 
			a[j] = -1;
		}
	}
	
	for(i = 0; i <= length; i ++) {
		
		if(a[i] != -1)
		number ++;
	} 
	 
	
	printf("%d", number);
	
	return 0;
}