#include <fstream>
#include <cmath>
using namespace std;
int N, M, D[1000002], L[1000002], poz;

ifstream f("orase.in");
ofstream g("orase.out");
int main()
{
    f >> M >> N;
    int mx = 0;
    for(int i = 1; i <= N; i++){
        f >> D[i] >> L[i];
        if(L[i] > mx) {
                mx = L[i];
                poz = i;
              }
        }
        int maxDist = 0;
    for(int i = 1; i <= N; i++)
     if(i != poz && L[poz] + L[i] + abs(D[poz] - D[i]) > maxDist) maxDist = L[poz] + L[i] + abs(D[poz] - D[i]);

     g << maxDist;
    return 0;
}