#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
long long a[N], fac[40];
map<long long, long long> mp;
int main() {
  long long n, x;
  for (long long i = 1; i <= 31; ++i) fac[i] = 1 << i;
  scanf("%lld", &n);
  for (long long i = 0; i != n; ++i) {
    scanf("%lld", &a[i]);
    mp[a[i]]++;
  }
  long long ans = 0;
  for (long long i = 0; i != n; ++i)
    for (long long j = 1; j <= 31; ++j) {
      ans += mp[fac[j] - a[i]];
      if (fac[j] - a[i] == a[i]) ans--;
    }
  printf("%lld\n", ans / 2);
  return 0;
}
