#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], maxim1, maxim2, i, poz;

int main()
{
	fin >> M >> N;
	for(i = 1; i <= N; i ++)
	{
		fin >> D[i] >> L[i];
		D[i] = D[i] * 1000000 + L[i];
	}
	sort(D + 1, D + N + 1);
	for(i = 1; i <= N; i ++)
	{
		L[i] = D[i] % 1000000 ;
		D[i] = D[i] / 1000000 ;
	}

	for(i = 1; i <= N; i ++)
	{
		if(L[i] - D[i] > maxim1)
		{
			maxim1 = L[i] - D[i];
			poz = i;
		}
		if(L[i] + D[i] > maxim2 && i > poz)
			maxim2 = L[i] + D[i];
	}
	fout << maxim1 + maxim2;
	return 0;
}