#include <fstream>
#include <math.h>
std::ifstream f1("orase.in");
std::ofstream f2("orase.out");
int main()
{
	long m, n, i, l1, l2, d1, d2, max1, max2, t1, t2, total1, total2, lmin, lmax, min, max;
	f1>>m>>n;
	f1>>t1>>l1;
	f1>>t2>>l2;
	if (t1<t2)
	{
		min=t1;
		lmin=l1;
		max=t2;
		lmax=l2;
	}//if
	else
	{
		min=t2;
		lmin=l2;
		max=t1;
		lmax=l1;
	}//else
	if (l1>l2)
	{
		max1=l1;
		d1=t1;
		max2=l2;
		d2=t2;
	}//if
	else
	{
		max2=l1;
		d2=t1;
		max1=l2;
		d1=t2;		
  }//else
	for (i=3; i<=n; i++)
	{
		f1>>t1>>l1;
		if (l1>max1)
		{
			max2=max1;
			d2=d1;
			max1=l1;
  		d1=t1;
		}//if
		else
			if (l1>max2)
			{
    		max2=l1;
		    d2=t1;
			}//max2
		if (t1<min)
		{
			min=t1;
			lmin=l1;
		}//if
		if (t1>max)
		{
			max=t1;
			lmax=l1;
		}//if
	}//for i
	total1=max1+max2+abs(d1-d2);
	total2=lmin+lmax+max-min;
	if (total1>total2)
  	f2<<total1<<"\n";
	else
		f2<<total2<<"\n";
	f1.close();
	f2.close();
	return 0;
}//main