#include<fstream>
#include<limits.h>
#define lim 50002
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int n,m,i,idx,smax,d;
struct oras{
	int d,l;
};
oras v[lim];
bool cmp(oras a,oras b){
	return a.d<b.d;
}
int main (){
	f>>m>>n;
	for(i=1;i<=n;i++)
		f>>v[i].d>>v[i].l;
	sort(v+1,v+n+1,cmp);
	idx=1;
	smax=-INT_MAX;
	for(i=2;i<=n;i++){
		d=v[i].d-v[idx].d+v[i].l+v[idx].l;
		if(smax<d)
			smax=d;
		if(v[i].l>v[i].d-v[idx].d+v[idx].l)
			idx=i;
	}
	g<<smax<<"\n";
	return 0;
}