#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
struct oras {
	long d,l;
};
inline bool cmp (const oras a, const oras b) 
{
	if(a.d==b.d)
		return a.l>b.l;
	else return a.d<b.d;
}
int main ()
{
	long n,i,j,m,c,ca,l,max;
	oras v[50001];
	ifstream f("orase.in");
	ofstream g("orase.out");
	f>>m>>n;
	for(i=1;i<=n;i++)
		f>>v[i].d>>v[i].l;
	f.close();
	sort(v+1,v+n+1,cmp);
	c=v[1].l;
	ca=v[1].d;
	max=c;
	for(i=2;i<=n;i++) {
		l=c+v[i].d-ca;
		if((v[i].l+l)>max)
			max=v[i].l+l;
		if(v[i].l>l) {
			c=v[i].l;
			ca=v[i].d;
		}
	}
	g<<max;
	g.close();
	return 0;
}