#include<fstream.h>
#include<math.h>
 ifstream f("orase.in");
 ofstream g("orase.out");
 int d[5001],l[5001],i,j,aux,min;

int main()
{
 int n;
 int m;
 f>>m;
 f>>n;
 for(i=1;i<=n;i++)
 {
  f>>d[i];
  f>>l[i];
 }
 for(i=1;i<n;i++)
  for(j=i+1;j<=n;j++)
  if(d[i]>d[j])
  {
   aux=d[i];
   d[i]=d[j];
   d[j]=aux;
    aux=l[i];
   l[i]=l[j];
   l[j]=aux;

  }
 min=0;
 for(i=1;i<=n;i++)
 {
  for(j=1;j<i;j++)
   if(l[i]+d[i]+l[j]-d[j]>min)
    min=l[i]+l[j]+d[i]-d[j];
 }
 g<<min;
 f.close();
 g.close();
 return 0;
}