#include <bits/stdc++.h>
using namespace std;
int main() {
  vector<int> v;
  int s = 1;
  while (s <= 1e9) {
    s *= 2;
    v.push_back(s);
  }
  int n;
  cin >> n;
  multiset<long long> ms;
  set<long long> e;
  for (int i = 0; i < n; i++) {
    long long a;
    cin >> a;
    ms.insert(a);
    e.insert(a);
  }
  long long ans = 0;
  for (auto u : e) {
    for (int i = 0; i < v.size(); i++) {
      long long tmp = ms.count(v[i] - u);
      if (v[i] - u == u) {
        ans += (tmp * (tmp - 1));
        continue;
      }
      ans += tmp * ms.count(u);
    }
  }
  cout << ans / 2 << endl;
  return 0;
}
