#include<iostream.h>
#include<stdio.h>
#include<math.h>
long long i,x[100000],y[100000],m,n,j,d,max;
int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	cin>>m>>n;
	for(i=1;i<=n;i++)
		cin>>x[i]>>y[i];
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
		{
			d=abs(x[i]-x[j])+y[i]+y[j];
			if(max<d) max=d;
		}
	cout<<max;
	return 0;
}