#include <fstream>
#include <vector>
using namespace std;

  main()
{
    int m,nrOrase,i,v1,v2,h,lungimeMaxima=0,j;
    ifstream in ("orase.in");
    ofstream out ("orase.out");
    in>>m>>nrOrase;
    vector<int> distanta;
    vector<int> lungime;
    distanta.resize(nrOrase);
    lungime.resize(nrOrase);
    for (i=0;i<nrOrase;++i)
    {
        in>>distanta[i]>>lungime[i];
    }
    for (i=0;i<nrOrase;++i)
    {
        v1=distanta[i];
        v2=lungime[i];
        for (h=0;h<nrOrase;++h)
        {
            if (i!=h)
            {
                j=v1+lungime[h]+max(v2-distanta[h],distanta[h]-v2);
                lungimeMaxima=max(j,lungimeMaxima);
            }
        }
    }
    out<<lungimeMaxima;
    return 0;
}