#include <stdio.h>
long int D[50000],L[50000];

void quicksort(long int li,long int ls)
{
long int i,j,mij,aux;

i=li;
j=ls;
mij=D[(li+ls)/2];

do
  {
   while (D[i]<mij) ++i;
   while (D[j]>mij) --j;
   if (i<=j)
      {
       aux=D[i];
       D[i]=D[j];
       D[j]=aux;
       aux=L[i];
       L[i]=L[j];
       L[j]=aux;
       ++i;
       --j;
       }
}
while (i<=j);
if (li<j) quicksort(li,j);
if (i<ls) quicksort(i,ls);
}


int main()
{
freopen("orase.in","rt",stdin);

long int n,m;
scanf("%ld %ld",&m,&n);
long int i;
for (i=1;i<=n;++i)
    scanf("%ld %ld",&D[i], &L[i]);
quicksort(1,n);
long int maxim=0,q,j=1;
while (j<=n)
      {
       i=j+1;
	while (i<=n)
	      {
	       if (maxim<D[i]+L[i]+L[j]-D[j])
		   maxim=D[i]+L[i]+L[j]-D[j];
		   ++i;
		   }
		  ++j;
		    }
freopen("orase.out","wt",stdout);
printf("%ld\n",maxim);
return 0;
}