#include<fstream>
#include<iostream>
#include<algorithm>
using namespace std;

const int N = 50005;

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

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

int main () {
	ifstream aa("orase.in");
	ofstream ss("orase.out");
	int i,n,m,u,di,dmax;
	aa >> m >> n;
	for (i=1;i<n+1;++i) {
		aa >> v[i].x >> v[i].y;
	}
	sort(&v[1],&v[n+1],cmp);
	u = 1;
	dmax = 0;
	for(i=2 ; i<=n ; ++i)
	{
		di=v[i].y+v[u].y+v[i].x-v[u].x;
		if (di>dmax) {
			dmax=di;
		}
		if (v[i].y > v[u].y+v[i].x-v[u].x) {
			u=i;
		}
	}
	ss<< dmax;
	aa.close();
	ss.close();
	return 0;
}