#include<stdio.h>
#include<algorithm>

using namespace std;

const int N=1<<16;

struct punct
{
	int x,y;
};
punct v[N];

bool cmp(punct p, punct q)
{
	return p.x<q.x;
}


int main()
{
	int u,i,dc,dmax=-1,n,m;
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	
	scanf("%d %d",&m,&n);
	
	for(i=1;i<=n;++i)
		scanf("%d %d",&v[i].x,&v[i].y);
	sort(&v[1],&v[n+1],cmp);
	
	u=1;
	for(i=2;i<=n;++i)
	{
		dc=0;
		dc=v[i].y + v[u].y + v[i].x - v[u].x;
		if (dc>dmax) dmax=dc;
		
		if(v[i].y > v[u].y + v[i].x - v[u].x)
			u=i;
	}

	printf("%d",dmax);
}