#include<fstream>
using namespace std;

unsigned short n, v[9001], f[7001], tot;
int main()
{
	ifstream fin("medie.in");
	ofstream fout("medie.out");
	fin >> n;
	for (unsigned short i = 1; i <= n; ++i)
	{
		fin >> v[i];
		++f[v[i]];
	}
	for (unsigned short i = 1; i < n; ++i)
		for (unsigned short j = i + 1; j <= n; ++j)
		{
			--f[v[i]], --f[v[j]];
			if ((((v[i] + v[j]) >> 1) << 1) == v[i] + v[j])
				tot += f[(v[i] + v[j]) >> 1];
			++f[v[i]], ++f[v[j]];
		}
	fout << tot;
}