#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
const int NMax=50005;
const int MMax=1000005;

int D[MMax],L[MMax],M,N,sol;

void Read()
{
    fin>>M>>N;
    for(int i=1;i<=N;i++)
    {
        fin>>D[i]>>L[i];
    }
}


void Solve()
{
    for(int i=1;i<=N;++i)
    {
        for(int j=1;j<=N;++j)
        {
            int x=max(D[i]-D[j],D[j]-D[i]);

            sol=max(sol,L[i]+L[j]+x);
        }
    }
    fout<<sol<<"\n";
}

int main()
{
    Read();
    Solve();
    return 0;
}