#include <iostream>
#include <fstream>

std::fstream fin("medie.in", std::ios::in);
std::fstream fout("medie.out", std::ios::out);

static const int mxn = 9e3, mxfv = 7e3;

int n, v[1 + mxn], fv[1 + mxfv];

int main(){
	fin >> n;
  for (int i = 1; i <= n; ++i){
    fin >> v[i];
    ++fv[v[i]];
  }
  int ans(0);
  for (int i = 1; i <= n; ++i){
    for (int j = i + 1; j <= n; ++j){
      /* if (((v[i] + v[j]) & 1) ^ 1){ */
      if ((v[i] + v[j]) % 2 == 0){
        ans += fv[(v[i] + v[j]) / 2];
        if (v[i] == v[j]) ans -= 2;
      }
    }
  }
  fout << ans << '\n';
  return 0;
}