#include <fstream>

using namespace std;

int main() {
	int n;
	int* v;
	int r;

	// input
	ifstream f("medie.in");
	f >> n;
	v = new int[n];
	for (int i = 0; i < n; i++) {
		f >> v[i];
	}
	f.close();

	// solve
	r = 0;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			for (int k = j + 1; k < n; k++) {
				if (i != j && j != k && i != k && v[i] == (v[j] + v[k]) / 2.0) {
					r++;
				}
			}
		}
	}

	// output
	ofstream g("medie.out");
	g << r << '\n';
	g.close();

	// free memory
	delete[] v;

	return 0;
}