#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
map<long long, int> mp;
long long f[34];
int a[N];
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  for (int i = 0; i <= 30; i++) f[i] = 1 << i;
  sort(a, a + n);
  long long ans = 0;
  for (int i = 0; i < n - 1; i++) {
    for (int j = 0; j <= 30; j++) {
      int t = f[j] - a[i];
      int l = lower_bound(a + 1 + i, a + n, t) - a;
      if (a[l] == t) {
        int r = upper_bound(a + i + 1, a + n, t) - a;
        ans += r - l;
      }
    }
  }
  cout << ans << endl;
  return 0;
}
