#include <stdio.h>

const int MAX = 7000;

int f[1 + MAX];

int main() {

  freopen("medie.in", "r", stdin);
  freopen("medie.out", "w", stdout);

  int n;
  scanf("%d", &n);
  int a[1 + n];
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
    f[a[i]]++;
  }
  long long ans = 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])
          ans += 1LL * (f[a[i]] - 2);
        else
          ans += 1LL * f[(a[i] + a[j]) / 2];
      }
  printf("%lld\n", ans);

  fclose(stdin);
  fclose(stdout);

  return 0;
}