#include<stdio.h>
#include<stdlib.h>
#define N 50000
int m,n;
struct vec{
	int x,y;
}a[N];
void read()
{
	int i;
	scanf("%d%d",&m,&n);
	for(i=0;i<n;i++)
		scanf("%d%d",&a[i].x,&a[i].y);
}
int maxim(const void *a,const void *b)
{
	vec *aa,*bb;
	aa=(vec*)a;
	bb=(vec*)b;
	return aa->x-bb->x;
}
void solve()
{
	int i,max,max_f,poz=0;
	max=a[0].y-a[0].x;
	qsort(a,n,sizeof(a[0]),maxim);
	for(i=0;i<n;i++)
		if(a[i].y-a[i].x>max)
		{
			max=a[i].y-a[i].x;
			poz=i;
		}
	if(poz!=0)
	{
		max_f=a[0].y-a[0].x+max;
		for(i=1;i<n;i++)
			if(poz!=i&&a[i].y+a[i].x+max>max_f)
				max_f=a[i].y+a[i].x+max;
	}
	else
	{
		max_f=a[1].y-a[1].x+max;
		for(i=2;i<n;i++)
			if(a[i].y+a[i].x+max>max_f)
				max_f=a[i].y+a[i].x+max;
	}
	printf("%d\n",max_f);
}
int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	read();
	solve();
	return 0;
}