#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;

size_t N, M;
ifstream fin("orase.in");
ofstream fout("orase.out");

struct Oras{
    size_t d, l;


   static inline size_t distance(Oras const& A, Oras const& B){
        return A.l + B.l + abs(A.d - B.d);
   }

   friend void operator >>(ifstream& in, Oras& O){
       in >> O.d >> O.l;
   }
};

Oras v[50001];

int main()
{
    fin >> M >> N;

    for(size_t i = 1; i <= N; i++)
        fin >> v[i];

    size_t dmax = 0;

    for(size_t i = 1; i < N; i++)
        for(size_t j = 2; j <= N; j++)
            dmax = max(dmax, Oras::distance(v[i], v[j]));

    fout << dmax;
}