#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int spr,n,i,y,dmax,dc;
struct strada {
	int d,l;
};
strada a[50005];
bool cmp(strada x,strada y) {
	return(x.l>y.l);
}
int main() {
	f>>spr>>n;
	for (i=1;i<=n;i++) f>>a[i].d>>a[i].l;
	sort(a+1,a+n+1,cmp);
	dmax=0;
	y=1;
	for (i=2;i<=n;i++) {
		dc=a[i].d-a[y].d+a[i].l+a[y].l;
		if (dc>dmax) dmax=dc;
		if(a[i].l>a[y].l+a[i].d-a[y].d) y=i;
	}
	g<<dmax;
	return 0;
}