#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
void solve() {
  int n;
  cin >> n;
  vector<long long> v(n);
  for (int i = 0; i < n; i++) cin >> v[i];
  map<long long, long long> M;
  for (int i = 0; i < n; i++) M[v[i]]++;
  vector<long long> X;
  long long p = 1;
  for (int i = 0; i <= 34; i++) {
    X.push_back(p);
    p *= 2;
  }
  long long ans = 0;
  for (int i = 0; i < n; i++) {
    M[v[i]]--;
    for (int j = 0; j < X.size(); j++) {
      ans += M[X[j] - v[i]];
    }
  }
  cout << ans << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int tests = 1;
  while (tests--) {
    solve();
  }
}
