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

  main()
{
    int lngStradaPrinc,nrOrase,i,j,di,li,d=0,lungimeMaxima=0;
    ifstream in ("orase.in");
    ofstream out ("orase.out");
    in>>lngStradaPrinc>>nrOrase;
    vector<int> distanta;
    vector<int> lungime;
    distanta.resize(nrOrase);
    lungime.resize(nrOrase);
    for (i=0;i<nrOrase;++i)
    {
        in>>distanta[i]>>lungime[i];
        if (d < distanta[i]) {d = distanta[i];}
    }
    if (lngStradaPrinc >= d)
    {
        for (i=0;i<nrOrase;++i)
        {
            di=distanta[i];
            li=lungime[i];
            for (j = i+1; j < nrOrase; ++j)
            {
                d = distanta[j] - di;
                if (d < 0) { d = -d;}
                d += lungime[j] + li;
                if (lungimeMaxima < d) { lungimeMaxima = d; }
            }
        }
        out<<lungimeMaxima;
    }
    else
    {
        ;   // error message
    }
    return 0;
}