#include<fstream>
using namespace std;

int n,  f[7001], tot;
int main()
{
	ifstream fin("medie.in");
	ofstream fout("medie.out");
	fin >> n;
	for (int i = 1, aux; i <= n; ++i)
	{
		fin >> aux;
		++f[aux];
	}
	
	int pos;
	for (int i = 0; i <= 7000; ++i)
		if (f[i] != 0)
			for (int j = 0; j <= 7000; ++j)
				if (f[j] != 0)
				{
					pos = f[i]; --f[i]; pos *= f[j]; --f[j];
					if ((i + j) % 2 == 0)
						tot += pos * f[(i + j) / 2];
					++f[i], ++f[j];
				}
	fout << tot / 2;
}