#include <stdio.h>
#include <algorithm>
#define N 850
#define P 60030
using namespace std;
int n,v[N],ap[P],s[P];
int rezultat;
void citire()
{
	scanf("%d",&n);
	int i;
	for (i=1; i<=n; i++)
	{
		scanf("%d",&v[i]);
		ap[v[i]]++;
	}
	for (i=1; i<=60000; i++)
		ap[i]+=ap[i-1];
	sort(v+1,v+n+1);
}
void rezolvare()
{
	int i,j;
	for (i=1; i<=n-2; i++)
		for (j=i+1; j<=n-1; j++)
			rezultat+=ap[v[i]+v[j]]-j;
	printf("%d\n",rezultat);
}
int main()
{
	freopen("nrtri.in","r",stdin);
	freopen("nrtri.out","w",stdout);
	citire();
	rezolvare();
	return 0;
}