#include <stdio.h>
#include <algorithm>

using namespace std;

int n,sol;
pair <int,int> P[50111];

#define f first
#define s second

int main()
{
	int a,b,i,c,d;
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	scanf("%*d %d",&n);
	for(i=0;i<n;++i)
		scanf("%d %d",&P[i].f,&P[i].s);
	sort(P,P+n);
	a=P[0].f, b=P[0].s;
	for(i=1;i<n;++i)
	{
		c=P[i].f, d=P[i].s;
		sol=max(sol,c-a+b+d);
		b=max(b+c-a,d);
		a=c;
	}
	printf("%d\n",sol);
	return 0;
}