#include<fstream.h>
int i,n,m,ok=0,max,q,p,s1,s2;
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;}
  }

max=v[1].y+v[2].y+v[2].x-v[1].x;
p=1; q=2;


for(i=3;i<=n;i++)

  { s1=v[i].y+v[p].y+v[i].x-v[p].x;
    s2=v[i].y+v[q].y+v[i].x-v[q].x;


  if (s1>max)

      if (s2>s1)   { max=v[i].y+v[q].y+v[i].x-v[q].x;

		    p=q;    q=i;
		   }

      else    {
		 max=v[i].y+v[p].y+v[i].x-v[p].x;

		  q=i;
	       }
    else

   if(s2>max) { max=v[i].y+v[q].y+v[i].x-v[q].x;

		 p=q;    q=i;
	      }
  }



g<<max;

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