#include<fstream.h>
int i,n,m,ok=0,j,max,max1,max2,l1,l2,q,p,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;}
  }

max1=v[1].y; p=1;
l1=v[1].x;

 for(i=2;i<=n;i++)

   if(v[i].y>max1+v[i].x-l1) {max1=v[i].y;l1=v[i].x;p=i;}

    else { max1=max1+v[i].x-l1;

	   l1=v[i].x;
	 }

max2=v[n].y;  q=n;
l2=v[n].x;

 for(i=n-1;i>=p;i--)

   if(v[i].y>max2+l2-v[i].x) {max2=v[i].y; l2=v[i].x; q=i;}

    else  {max2=max2+l2-v[i].x;

	   l2=v[i].x;
	  }

max=v[p].y+v[q].y+v[q].x-v[p].x ;
g<<max;


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