#include<stdio.h>
#include<algorithm>
const int N=50001;
using namespace std;
struct oras
{
	int x,y;
};

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