#include <bits/stdc++.h>
using namespace std;
int dx[] = {-1, -1, -1, 0, 0, 1, 1, 1};
int dy[] = {-1, 0, 1, -1, 1, -1, 0, 1};
int xChange[] = {0, 1, 0, -1};
int yChange[] = {1, 0, -1, 0};
const int N = 100001;
int n, a[N];
long long power[31], ans;
int main() {
  ios::sync_with_stdio(false), cin.tie(NULL);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  power[0] = 1;
  for (int i = 1; i <= 30; i++) power[i] = power[i - 1] * 2;
  sort(a, a + n);
  for (int i = 0; i < n; i++) {
    for (int j = 1; j <= 30; j++) {
      long long dif = abs(a[i] - power[j]);
      int up, lo;
      up = upper_bound(a + i + 1, a + n, dif) - a;
      lo = lower_bound(a + i + 1, a + n, dif) - a;
      ans += (up - lo);
    }
  }
  cout << ans;
  return 0;
}
