#include <bits/stdc++.h>
#define x first
#define y second

using namespace std;


int n, f[7002], v[9002], i, j;
long long ans;


int main()
{

  ifstream fin ("medie.in");
  ofstream fout ("medie.out");
  fin >> n;
  for (int i = 1; i <= n; i++) {
    fin >> v[i];
    f[v[i]]++;
  }
  for (i = 1; i <= n; i++)
    for (j = i + 1; j <= n; j++) {
      if ((v[i] + v[j]) % 2 == 1)
        continue;
      if (v[i] == v[j]) {
        ans += f[v[i]] - 2;
      }
      else {
        ans += f[(v[i] + v[j]) / 2];
      }
    }
  fout << ans;
  return 0;
}