#include <fstream>
#include <algorithm>

using namespace std;

#define N 1<<16
struct rr
{int d,l;} v[N];

bool comp(rr x, rr y)
{
	return x.d<y.d;
}

ifstream in("orase.in");
ofstream out("orase.out");
int main()
{
	int i0,dmax=0,i,m,n;
	in>>m>>n;
	for(i=1;i<=n;++i)
		in>>v[i].d>>v[i].l;
	sort(v+1,v+n+1,comp);
	i0=v[1].l;
	for(i=2;i<=n;++i)
	{
		if(v[i].l+v[i0].l+v[i].d-v[i0].d > dmax)
			dmax=v[i].l+v[i0].l+v[i].d-v[i0].d;
		if(v[i].l > v[i0].l+v[i].l-v[i0].d)
			i0=i;
	}
	out<<dmax;
	return 0;
}