#include <bits/stdc++.h>
using namespace std;
const int kMaxN = 100010;
const int kMaxA = 1000000010;
int a[kMaxN];
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long long n;
  cin >> n;
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n);
  long long ans = 0;
  for (long long x = 1; (1LL << x) < 2 * kMaxA; x++) {
    long long val = (1 << x);
    for (long long i = 0; i < n; i++) {
      long long num = upper_bound(a + i + 1, a + n, val - a[i]) -
                      lower_bound(a + i + 1, a + n, val - a[i]);
      ans += num;
    }
  }
  cout << ans << endl;
  return 0;
}
