#include <fstream>
#include <iostream>

#define sFin "orase.in"
#define sFout "orase.out"
#define nMax 1000069
#define abs(x) (x<0?-1*x:x)
#define s(x) (x[0]+x[1])

using namespace std;

int n,m,sir[nMax][2];

void mainLoop(){
	ifstream fin(sFin);
	ofstream fout(sFout);

	fin >> m >> n;
	int sol = -1*int(2e9);

	for (int i=0;i<n;i++){
		int x, y;
		fin >> x >> y;
		sir[i][0] = x;
		sir[i][1] = y;
	}

	for (int i=0;i<n;i++)
		for (int j=0;j<n;j++)
			if (sir[i][1] + sir[j][1] + abs(sir[i][0]-sir[j][0]) > sol)
				sol = sir[i][1] + sir[j][1] + abs(sir[i][0]-sir[j][0]);

	fout << sol;

	fout.close(), fin.close();
}

int main(){
	mainLoop();
	return 0;
}