#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,j,m,n,sc,smax=-100;
	f>>m>>n;
	for (i=1; i<=n; i++)
	{
		f>>v[i].x>>v[i].y;
	}
	sort(&v[1],&v[n+1],cmp);
	for (i=1; i<=n-1; i++)
	{
		sc=0;
		for (j=i+1; j<=n; j++)
		{
			sc=v[i].y+v[j].x-v[i].x+v[j].y;
			if (sc>smax)
				smax=sc;
		}
	}
	g<<smax;
	return 0;
}