#include <fstream>
using namespace std;

#define MAX_ORASE   50000

int main()
{
    int lngStradaPrinc,nrOrase,d=0,lungimeMaxima=0;
    int distanta[MAX_ORASE];
    int lungime [MAX_ORASE];
    int* curD;
    int* curL;
    int* lastD;
    ifstream in ("orase.in");
    ofstream out ("orase.out");
    in>>lngStradaPrinc>>nrOrase;
    if (nrOrase <= MAX_ORASE)
    {
        lastD = distanta + nrOrase;
        curD  = distanta;
        curL  = lungime;
        while (curD < lastD)
        {
            in >> *curD >> *curL;
            ++curD;
            ++curL;
        }
        if (lngStradaPrinc >= d)
        {
            int* curD2;
            int* curL2;
            curD  = distanta;
            curL  = lungime;
            while (curD < lastD)
            {
                curD2 = curD + 1;
                curL2 = curL + 1;
                while (curD2 < lastD)
                {
                    d = *curD - *curD2;
                    if (d < 0) { d = -d;}
                    d += *curL + *curL2;
                    if (lungimeMaxima < d) { lungimeMaxima = d; }
                    ++curD2;
                    ++curL2;
                }
                ++curD;
                ++curL;
            }
            out<<lungimeMaxima;
        }
        else
        {
            out<<"Distanta prea mare";   // error message
        }
    }
    else
    {
        out<<"Nr orase prea mare";   // error message
    }
    return 0;
}