#include <stdio.h>
#include <algorithm>
# define N 50010
using namespace std;
struct orase{ int d,l;};
orase v[N];
int f1,f2,i,drum,n,m;
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 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)
	{
		f1=v[i].d-v[i-1].d+v[i].l-v[i-1].l;
		f2=v[i].l+v[i-1].l+v[i].d-v[i-1].d;
		if(drum+f1>f2) 
			drum+=f1;
		else 
			drum=f2;
		if(drum>rez) 
			rez=drum;
	}
	printf("%d\n", rez);
}
int main()
{
	scan();
	solve();
	return 0;
}