#include <stdio.h>

#define NMAX 50000

long int n,m,i,j,aux,maxim,q;
long int D[NMAX],L[NMAX];

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

scanf("%ld %ld", &m, &n);

for (i=1;i<=n;i++)
    scanf("%ld %ld", &D[i], &L[i]);

for (i=1;i<=n-1;++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;
	      }
maxim=0;
j=1;
while (j<=n)
      {
       i=j+1;
	while (i<=n)
	      {
		 q=D[i]+L[i]+L[j]-D[j];
	       if (maxim<q)
		   maxim=q;
		   ++i;
		   }
		  ++j;
		    }
printf("%ld\n",maxim);
return 0;
}