#include <cstdio>

const int MAXN = 9e3;

int v[MAXN], cnt[MAXN];

int main() {
  int n, ans, m;
  FILE *f = fopen("medie.in", "r");
  fscanf(f, "%d", &n);
  for (int i = 0; i < n; ++i) {
    fscanf(f, "%d", &v[i]);
    ++cnt[v[i]];
  }
  fclose(f);
  ans = 0;
  for (int i = 0; i < n; ++i) {
    for (int j = i + 1; j < n; ++j) {
      if (!((v[i] + v[j]) & 1)) {
        m = (v[i] + v[j]) >> 1;
        ans += cnt[m];
        if (v[i] == m) {
          --ans;
        }
        if (v[j] == m) {
          --ans;
        }
      }
    }
  }
  f = fopen("medie.out", "w");
  fprintf(f, "%d\n", ans);
  fclose(f);
  return 0;
}