#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int x;
  long long y = 0;
  cin >> n;
  long t;
  vector<int> a;
  map<int, int> c;
  for (int i = 0; i < n; i++) {
    cin >> t;
    a.push_back(t);
    c[t]++;
  }
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < 31; j++) {
      x = pow(2, j) - a[i];
      if ((x != a[i]) && (c[x] > 0)) {
        y += c[x];
      }
      if ((x == a[i]) && (c[x] > 1)) {
        y += c[x] - 1;
      }
    }
    c[a[i]] -= 1;
  }
  cout << y;
}
