#include <stdio.h>
#include <algorithm>
using namespace std;
# define N 50010
int form1,form2,i,drum,n,m,best,a;
struct orase{ int d,l;};
orase v[N];
void scan()
{
	freopen("orase.in", "r",stdin);
	freopen("orase.out", "w",stdout);
	scanf("%d%d", &m,&n);
	for(int j=1;j<=n;++j)
		scanf("%d%d", &v[j].d,&v[j].l);
}
bool comp(const orase &x, const orase &y)
{  if(x.d<y.d) return true; return false;} 

void formula(){ form1=v[i].d-v[i-1].d+v[i].l-v[i-1].l;}
void formula2(){form2=v[i].l+v[i-1].l+v[i].d-v[i-1].d;}	

void solve()
{
	sort(v+1,v+n+1, comp);
	drum=v[2].d-v[1].d+v[2].l+v[1].l;
	int rez=drum;
	for(i=3;i<=n;++i)
	{
		formula(); formula2();
		if(drum+form1>form2) 
			drum+=form1;
		else 
			drum=form2;
		if(drum>rez) 
			rez=drum;
	}
	printf("%d\n", rez);
}
int main()
{
	scan();
	solve();
	return 0;
}