#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
struct oras
{
	int d,l;
};
bool comp(const oras & x, const oras & y)
{
	return x.d<y.d;
}
oras a[1<<16];
int main()
{
	int m,n,d,dmax=0,i;
	ifstream f("orase.in");
	ofstream g("orase.out");
	f>>m;
	f>>n;
	for(i=1; i<=n; i++)
	{
		f>>a[i].d;
		f>>a[i].l;
	}
	sort(a+1,a+1+n,comp);
	d=a[1].d+a[i].l;
	for(i=1; i<=n; i++)
	{
		d=d+a[i].d-a[i-1].d;
		if(d+a[i].l>dmax)
			dmax=d+a[i].l;
		if(d<a[i].l)
			d=a[i].l;
	}
	g<<dmax;
	return 0;
}