#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void ZZ(const char* name, Arg1&& arg1) {
  cerr << name << " = " << arg1 << "\n";
}
template <typename Arg1, typename... Args>
void ZZ(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " = " << arg1;
  ZZ(comma, args...);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  long long n;
  cin >> n;
  map<long long, long long> m;
  for (long long i = (0); i < (n); i++) {
    long long x;
    cin >> x;
    m[x]++;
  }
  long long ans = 0, same = 0;
  for (auto& [x, y] : m) {
    for (long long i = (0); i < (32); i++) {
      long long req = (1 << i) - x;
      if (req <= 0) continue;
      if (m.count(req) && req != x) ans += y * m[req];
      if (req == x) same += (y * (y - 1)) / 2;
    }
  }
  ans /= 2;
  ans += same;
  cout << ans;
}
