#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");

int n, v[9001], sol, fr[7001];


int main()
{
	int i, j, x, y;
	fin >> n;
	for (i = 1; i <= n; i++)
	{
		fin >> v[i];
		fr[v[i]]++;
	}
	for (i = 1; i <= n; i++)
		for (j = i + 1; j <= n; j++)
			if ((v[i] + v[j]) % 2 == 0)
			{
				x = (v[i] + v[j]) / 2;
				sol += fr[x];
			}
	fout << sol;
	return 0;
}