#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
ifstream fin("medie.in");
ofstream fout("medie.out");
int n, v[9005], fv[7005], rez;
int main()
{
  fin >> n;
  for (int i = 1; i <= n; ++i)
  {
    fin >> v[i];
    ++fv[v[i]];
  }
  for (int i = 1; i <= n; ++i)
    for (int j = i + 1; j <= n; ++j)
      if ((v[i] + v[j]) % 2 == 0)
      {
        rez += fv[(v[i] + v[j]) / 2];
        if (v[i] == v[j])
          rez -= 2;
      }
  fout << rez;
  return 0;
}