#include <iostream>
#include <fstream>
#define INFILE "orase.in"
#define OUTFILE "orase.out"
#define max_N 51000

using namespace std;

ifstream fin(INFILE);
ofstream fout(OUTFILE);

long int M, N, D[max_N], L[max_N], j, maxim1, maxim2, i, poz, maxim;

int main()
{
	fin >> M >> N;
	for(i = 1; i <= N; i ++)
	{
		fin >> D[i] >> L[i];
		D[i] = D[i] * 100000 + L[i];
	}
	sort(D + 1, D + N + 1);
	for(i = 1; i <= N; i ++)
	{
		L[i] = D[i] % 100000 ;
		D[i] = D[i] / 100000 ;
	}
	
	for(i = 2; i <= N; i ++)
	{
		
		for(j = 1; j < i ; j ++)
			if(abs(L[j] - D[j]) + L[i] + D[i] > maxim)
				maxim = abs(L[j] - D[j]) + L[i] + D[i];
		
	}
	

	fout << maxim;
	return 0;
}