#include<stdio.h>
#include<algorithm>
using namespace std;
struct oras
{
	int x,y;
};
const int N=1<<16;
oras v[N];
bool comp(oras a,oras b)
{
	return a.x<b.x;
}
int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	int x0,y0,n,i,m,dc,dmax;
	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,comp);
	x0=v[1].x;
	y0=v[1].y;
	dmax=0;
	for(i=2;i<=n;i++)
	{
		dc=y0+v[i].y+v[i].x-x0;
		if (dc>dmax) dmax=dc;
		if(v[i].y>y0+v[i].x-x0)
		{
			x0=v[i].x;
			y0=v[i].y;
		}
	}
	printf("%d",dmax);
	return 0;
}