#include <fstream>
#include <cmath>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");

struct strada
{
    int di,li;
}v[50001];

int main()
{
    int dmax,d,distanta,i,j,n,m;
    fin>>m>>n;
    dmax=0;
    distanta=0;
    for(i=1;i<=n;i++)
       fin>>v[i].di>>v[i].li;
    for(i=1;i<n;i++)
      for(j=i+1;j<=n;j++)
         {d=abs(v[i].di-v[j].di);
         distanta=d+v[i].li+v[j].li;
        if(distanta>dmax)
            dmax=distanta;
         }
   fout<<dmax;
    return 0;
}