#include <bits/stdc++.h>
using namespace std;
long long ans = 0;
long long a[100050];
int n;
inline void get_ready() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  sort(a + 1, a + n + 1);
}
inline void solve() {
  for (int i = 1; i <= 32; i++) {
    long long cnt = 1 << i;
    if (cnt > 2 * a[n]) break;
    for (int j = 1; j <= n - 1; j++) {
      int x1 = lower_bound(a + j + 1, a + n + 1, cnt - a[j]) - a;
      int x2 = upper_bound(a + j + 1, a + n + 1, cnt - a[j]) - a;
      if (x1 == n + 1) continue;
      ans += x2 - x1;
    }
  }
  cout << ans << endl;
}
int main() {
  std::ios::sync_with_stdio(0);
  get_ready();
  solve();
  return 0;
}
