#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
int a[9005],cate[7005],v[7005];
int main()
{
	ifstream in("medie.in");
	ofstream out("medie.out");
	int i, n, j;
	long long tot=0;
	in >> n;
	for (i = 1; i <= n; i++)
	{
		in >> a[i];
		cate[a[i]]++;
	}
	for (i = 1; i <= n; i++)
	{
		for (j = i + 1; j <= n; j++)
		{
			if ((a[i] + a[j])%2==0)
			v[(a[i] + a[j]) / 2]++;
		}
	}
	for (i = 1; i <= 7000; i++)
	{
		tot += cate[i] * v[i]-(cate[i])*(cate[i]-1);
	}
	out << tot;
}