#include <fstream>

using namespace std;

ifstream cin("medie.in");
ofstream cout("medie.out");

int f[7001],v[9001];

signed main() {
  int n, i, j, comp;
  long long rez = 0;
  cin >> n;
  for (i = 1; i <= n; i++) {
    cin >> v[i];
    f[v[i]]++;
  }
  for (i = 1; i <= n; i++) {
    f[v[i]]--;
    for (j = 1; j < v[i]; j++) {
      if (2 * v[i] - j > 7000)
        continue;
      rez += 1LL * (f[j] * f[2 * v[i] - j]);
    }
    rez += 1LL * (f[v[i]] * (f[v[i]] - 1)) / 2;
    f[v[i]]++;
  }
  cout << rez;
  return 0;
}