#include<fstream>
#include<algorithm>


using namespace std;
struct oras
{
	int x,y;
};
const int N = 1<<16;

ifstream f("orase.in");
ofstream g("orase.out");
	
oras v[N];

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

int main()
{
	int i,m,n,d,max=-100,u;
	f>>m>>n;
	for (i=1; i<=n; i++)
	{
		f>>v[i].x>>v[i].y;
	}
	sort(&v[1],&v[n+1],cmp);
	u=1;
	for (i=2; i<=n; i++)
	{
		d=v[i].x-v[u].x+v[i].y+v[u].y;
		if (d>max) 
			max=d;
		if (v[i].y>v[i].x-v[u].x+v[u].y)
			u=i;
	}
	g<<max;
	return 0;
}