#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  vector<long long> s;
  for (long long k = 1; k <= 1e12; k *= 2) {
    s.push_back(k);
  }
  long long ans = 0;
  map<long long, long long> m;
  for (auto x : s) {
    m.clear();
    for (int i = 0; i < n; i++) {
      ans += m[x - a[i]];
      m[a[i]]++;
    }
  }
  cout << ans << "\n";
}
