#include <stdlib.h>
#include <fstream>
using namespace std;

#define MAX_ORASE   50000

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