#include <fstream>
using namespace std;

#define in "medie.in"
#define out "medie.out"
#define N 9005
#define MAX 7005

short v[N], n, f[MAX];
long long sol;

int main ()
{
	ifstream fin (in);
	fin >> n;
	for (int i = 0; i < n; ++i)
		fin >> v[i];
	for (int i = 0; i < n - 1; ++i)
		for (int j = i + 1; j < n; ++j)
			if (!((v[i] + v[j]) & 1)) {
				f[(v[i] + v[j]) >> 1]++;
				if (v[i] == (v[i] + v[j]) >> 1)
					sol --;
				if (v[j] == (v[i] + v[j]) >> 1)
					sol --;
			}
	for (int i = 0; i < n; ++i)	
		sol += f[v[i]];
	ofstream fout (out);
	fout << sol;
	fcloseall();
	return 0;
}