#include <stdio.h>
int main()
{long long n,m,l[50000],d[50000],max,i,j;
 
 freopen("orase.in","r",stdin);
 freopen("orase.out","w",stdout);
 scanf("%lld %lld",&m,&n);
 for (i=1;i<=n;i++)
	 scanf("%lld %lld",&d[i],&l[i]);
 for (i=1;i<n;i++)
	 for (j=i+1;j<=n;j++)
		 if (d[i]>d[j]) 
		 {m=d[i];d[i]=d[j];d[j]=m;
		  m=l[i];l[i]=l[j];l[j]=m;
		 }
 for (i=2,j=1,max=-1;i<=n;i++)
    {if (l[i]+d[i]+l[j]-d[j]>max) max=l[i]+d[i]+l[j]-d[j];
     if (l[i]-d[j]>l[j]-d[j]) j=i;
	}
 printf("%lld",max);
return 0;}