#include<stdio.h>
#include<algorithm>
using namespace std;
struct oras
{
	int d,l;
};
bool comp(const oras&x, const oras&y)
{
	return x.d<y.d;
}
int main()
{
	int m,n,i;
	oras v[1<<16];
	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].d,&v[i].l);
	}
	sort(v+1,v+n+1,comp);
	int x=v[1].l,dmax=0;
	for(i=2;i<=n;++i)
	{
		x+=v[i].d-v[i-1].d;
		if(dmax<x+v[i].l) dmax=x+v[i].l;
		if(x<v[i].l)x=v[i].l;
	}
	printf("%d",dmax);
	return 0;
}