#include<cstdio>
#include<algorithm>

using namespace std;

const int N=50005;

struct orase
{
	int d,l;
};

int lung,n;
orase v[N];

bool comp(orase a,orase b)
{
	return a.d<b.d;
}

void citire()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	scanf("%d%d",&lung,&n);
	for (int i=1;i<=n;++i)
		scanf("%d%d",&v[i].d,&v[i].l);
	sort(v+1,v+n+1,comp);
}

void rez()
{
	int u=1,distmax=0,dist;
	for (int i=2;i<=n;++i)
	{
		dist=v[i].l+v[u].l+v[i].d-v[u].d;
		if (dist>distmax)
			distmax=dist;
		if (v[i].l>v[u].l+v[i].d-v[u].d)
			u=i;
	}
	printf("%d\n",distmax);
}

int main()
{
	citire();
	rez();
	return 0;
}