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

int a[9010], fr[7010];

int main()
{
	int n, i, j, s, sol = 0;
	fin >> n;
	for(i = 1 ; i <= n ; i++)
	{
		fin >> a[i];
		fr[a[i]]++;
	}
	for(i = 1 ; i <= n ; i++)
	{
		for(j = i + 1 ; j <= n ; j++)
		{
			s = a[i] + a[j];
			if(s % 2 == 0)
			{
				sol += fr[s / 2];
				if(a[i] == s / 2)
					sol -= 2;
			}
		}
	}
	fout << sol << "\n";
}