#include <fstream>
#include <algorithm>

using namespace std;

ifstream in("orase.in");
ofstream out("orase.out");

struct oras{
	int d;
	int l;
};

bool comp(const oras &x, const oras &y){
	return x.d<y.d;
}

oras v[1<<16];

int main(){
	int n,m,i;
	in>>m>>n;
	for(i=0;i<n;i++){
		in>>v[i].d;
		in>>v[i].l;
	}
	sort(v,v+n,comp);
	int d=v[0].l+v[1].d,dmax=0;
	for(i=1;i<n;i++){
		if(d+v[i].l>dmax){
			dmax=d+v[i].l;
		}
		if(d+v[i+1].d-v[i].d>v[i].l+v[i+1].d-v[i].d){
			d=d+v[i+1].d-v[i].d;
		}else{
			d=v[i].l+v[i+1].d-v[i].d;
		}
	}
	out<<dmax;
	return 0;
}