#include<stdio.h>
#include<algorithm>
using namespace std;
struct oras
{
	int x;
	int lungime;
};
oras v[50005];
bool comp(oras a,oras b)
{
	if(a.x<b.x)
		return true;
	return false;
}
int modul(int x)
{
	if(x<0)
		return -x;
	return x;
}
int m,n;
int main()
{
	int i;
	FILE *f=fopen("orase.in","r");
	FILE *g=fopen("orase.out","w");
	fscanf(f,"%d",&m);
	fscanf(f,"%d",&n);
	for(i=1;i<=n;i++)
	{
		fscanf(f,"%d",&v[i].x);
		fscanf(f,"%d",&v[i].lungime);
	}
	sort(v+1,v+n,comp);
	oras maxim;
	int poz1,poz2;
	maxim.x=v[1].x;
	maxim.lungime=v[1].lungime;
	poz1=1;
	for(i=2;i<=n;i++)
	{
		if((v[i].x-maxim.x+maxim.lungime)<v[i].lungime)
		{
			maxim.x=v[i].x;
			maxim.lungime=v[i].lungime;
			poz1=i;
		}
	}
	maxim.x=v[n].x;
	maxim.lungime=v[n].lungime;
	poz2=n;
	for(i=n;i>=1;i--)
	{
		if(v[i].lungime>(maxim.x-v[i].x+maxim.lungime))
		{
			maxim.x=v[i].x;
			maxim.lungime=v[i].lungime;
			poz2=i;
		}
	}
	if(poz1!=poz2)
	{
		fprintf(g,"%d",v[poz1].lungime+v[poz2].lungime+modul(v[poz1].x-v[poz2].x));
	}
	else
	{
		int d=0,curent;
		for(i=1;i<poz1;i++)
		{
			curent=v[i].lungime+v[poz1].lungime+modul(v[poz1].x-v[i].x);
			if(curent>d)
				d=curent;
		}
		for(i=poz1+1;i<=n;i++)
		{
			curent=v[i].lungime+v[poz1].lungime+modul(v[poz1].x-v[i].x);
			if(curent>d)
				d=curent;
		}
		fprintf(g,"%d",d);
	}
}