#include<stdio.h>
#include<algorithm>
using namespace std;
struct oras
{
	int d,l;
};

bool cmp(oras x,oras y)
{
	if(x.d<y.d)
		return true;
	return false;
}

oras v[50001];

int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	int dc,dmax=0,i,u,n,m;
	scanf("%d%d",&m,&n);
	for(i=1;i<=n;i++)
		scanf("%d%d",&v[i].d,&v[i].l);
	
	sort(&v[1],&v[n+1],cmp);
	
	u=1;
	for(i=2;i<=n;i++)
	{
		dc=v[u].l+v[i].l+v[i].d-v[u].d;
		if(dc>dmax)
			dmax=dc;
		
		if(v[i].l>v[u].l+v[i].d-v[u].d)
			u=i;
	}
	printf("%d",dmax);
	return 0;
}