#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
int n,x[1000],s,q,w,nr;
int main()
{
	freopen("nrtri.in","r",stdin);
	freopen("nrtri.out","w",stdout);
	scanf("%d",&n);
	int i,j;
	for(i=1;i<=n;i++)
		scanf("%d",&x[i]);
	sort(x+1,x+n+1);
	
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
		{
			q=j+1;
			w=n;
			s=x[i]+x[j];
			while(q<w)
			{
				if(x[q+(w-q)/2]==s)
					q=w=q+(w-q)/2;
				else
					if(x[q+(w-q)/2]<s)
						w=q+(w-q)/2;
					else
						q=q+(w-q)/2;
			}
			nr+=w-j;
		}
	printf("%d",nr);
	
	return 0;
}