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