#include <fstream>

#define NMAX 9005
#define DIM 7005

using namespace std;

ifstream f ("medie.in");
ofstream g ("medie.out");

int c[DIM] , v[NMAX];
int n;

long long sol;

int main() {
    f >> n;
    for (int i = 1; i <= n; ++i) {
        f >> v[i];
        ++c[v[i]];
    }

    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j < i; ++j) {
            int aux = v[i] + v[j];
            if (aux % 2) {
                continue;
            }

            aux /= 2;

            sol += (long long) c[aux];
            if (v[i] == aux) {
                --sol;
            }

            if (v[j] == aux) {
                --sol;
            }
        }
    }

    g << sol;
    return 0;
}