#include <cstdio>
using namespace std;
const int NMAX = 10005;
int vf[NMAX];
int a[NMAX];

int main() {
  int n;
  freopen("medie.in", "r", stdin);
  freopen("medie.out", "w", stdout);
  scanf("%d", &n);
  for(int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
    vf[a[i]]++;
  }
  long long sol = 0;
  for(int i= 1; i <= n; i++) {
    for(int j = i + 1; j <= n; j++) {
      if(a[i] % 2 == a[j] % 2) {
        if(a[i] == a[j]) {
          sol += 1LL * (vf[a[i]] - 2);
        }
        else {
          sol += 1LL * vf[(a[i] + a[j]) / 2];
        }
      }
    }
  }
  printf("%lld\n", sol);
  return 0;
}