#include <bits/stdc++.h>

using namespace std;

ifstream fin( "medie.in" );
ofstream fout( "medie.out" );

const int DIM = 9005; 

int f[DIM];
int v[DIM];

int main() {
  int n;
  
  fin >> n;
  for ( int i = 1; i <= n; ++i ) {
	fin >> v[i];
    ++f[v[i]];
  }
  int res = 0;
  for ( int i = 1; i <= n; ++i ) {
	for ( int j = i + 1; j <= n; ++j ) {
	  int val = (v[i] + v[j]) / 2;
	  if ( (v[i] + v[j]) % 2 == 0 ) {
		res += f[val] - (v[i] == val) * 2;
	  }
	}
  }
  fout << res;
  fin.close();
  fout.close();
  return 0;
}