#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
long long n, a[N];
void init() {
  cin >> n;
  for (int i = 0; i < n; ++i) cin >> a[i];
  sort(a, a + n);
}
long long check(long long now) {
  long long ans = 0;
  map<long long, long long> ma;
  map<long long, long long>::iterator it = ma.begin();
  for (long long i = 0; i < n; ++i) {
    if (ma.count(now - a[i]) > 0) {
      ans += ma[now - a[i]];
    }
    ++ma[a[i]];
  }
  return ans;
}
void solve() {
  long long ans = 0, pow = 1;
  for (long long i = 0; i < 63; ++i) {
    if (i != 0) pow = pow << 1;
    ans += check(pow);
  }
  cout << ans << endl;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int __ = 1;
  while (__--) {
    init();
    solve();
  }
}
