#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
class InParser {
private:
	FILE *fin;
	char *buff;
	int sp;
	char read_ch() {
		++sp;
		if (sp == 4096) {
			sp = 0;
			fread(buff, 1, 4096, fin);
		}
		return buff[sp];
	}
public:
	InParser(const char* nume) {
		fin = fopen(nume, "r");
		buff = new char[4096]();
		sp = 4095;
	}
	InParser& operator >> (int &n) {
		char c;
		while (!isdigit(c = read_ch()) && c != '-');
		int sgn = 1;
		if (c == '-') {
			n = 0;
			sgn = -1;
		} else {
			n = c - '0';
		}
		while (isdigit(c = read_ch())) {
			n = 10 * n + c - '0';
		}
		n *= sgn;
		return *this;
	}
	InParser& operator >> (long long &n) {
		char c;
		n = 0;
		while (!isdigit(c = read_ch()) && c != '-');
		long long sgn = 1;
		if (c == '-') {
			n = 0;
			sgn = -1;
		} else {
			n = c - '0';
		}
		while (isdigit(c = read_ch())) {
			n = 10 * n + c - '0';
		}
		n *= sgn;
		return *this;
	}
};
InParser fin("medie.in");
ofstream fout("medie.out");
int n, v[9005], fv[7005], rez;
int main()
{
  ios::sync_with_stdio(false);
  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 && fv[(v[i] + v[j]) / 2])
      {
        rez += fv[(v[i] + v[j]) / 2];
        if (v[i] == v[j])
          rez -= 2;
      }
  fout << rez;
  return 0;
}