#include<fstream>
#include<algorithm>
#define MaxN 50005

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

struct casa	{
	int l,d;
};
casa v[MaxN];
int i,j=1,n,m,D,dis;

int cmp(casa a, casa b)
{	if(a.d==b.d)return a.l<b.l;
	return a.d<b.d;}

int main()
{	fin>>m>>n;
	for(i=1;i<=n;i++)	
		fin>>v[i].d>>v[i].l;
	sort(v+1,v+1+n,cmp);
	dis=v[1].l-v[1].d;j=1;
	for(i=2;i<=n;i++)
	{	if(v[i].l+v[i].d+v[j].l-v[j].d>D) D=v[i].l+v[i].d+dis;
		if(v[i].l-v[i].d>dis) 	
		{	dis=v[i].l-v[i].d;j=i;}
	}
	fout<<D<<'\n';
	return 0;
}