#include<cstdio>
#include<algorithm>
using namespace std;
struct PPOZ
{int x, y;};
bool comp(PPOZ a, PPOZ b)
{
	if(a.x<b.x)
		return true;
	if(a.x>b.x)
		return false;
	return (a.y>b.y);
}
int d,mmax,m,n,i;
PPOZ v[50005];
int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	scanf("%d%d",&m,&n);
	for(i=1;i<=n;i++)
		scanf("%d%d",&v[i].x,&v[i].y);
	sort(v+1,v+1+n,comp);
	d=v[1].y;
	mmax=0;
	for(i=2;i<=n;i++)
	{
		d+=v[i].x-v[i-1].x;
		if(d+v[i].y>mmax)
			mmax=d+v[i].y;
		if(v[i].y>d)
			d=v[i].y;
	}
	printf("%d",mmax);
	return 0;
}