#include<fstream.h>
using name std;
int i,n,m,ok=0,j,max=0,l,d,s;
struct sir {int x,y;};
sir v[50010],aux;
int main()
{
ifstream f("orase.in");
ofstream g("orase.out");

f>>m>>n;

 for(i=1;i<=n;i++)

    f>>v[i].x>>v[i].y;


while(!ok)
 {
  ok=1 ;

   for(i=1;i<n;i++)

     if(v[i].x>v[i+1].x) {aux=v[i]; v[i]=v[i+1]; v[i+1]=aux; ok=0;}
  }


for(i=1;i<n;i++)
 for(j=i+1;j<=n;j++)

  { s=v[j].x-v[i].x+v[i].y+v[j].y;
    if(s>max) max=s;
  }

g<<max;

f.close();
g.close();
return 0;
}