#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>
#include <algorithm>
#include <iomanip>
#include <math.h>
#include <string.h>
#include <cstdio>
#include <tuple>
#include <numeric>
using namespace std; using ll = long long; using ld = long double;  using pll = pair<ll, ll>;
using vl = vector<ll>; using vll = vector<vl>; using vpll = vector<pll>;
using vs = vector<string>; using tll = tuple<ll, ll, ll>; using vtll = vector<tll>;
const ld PI = 3.1415926535897932;
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define mp make_pair
#define mt make_tuple
void in() {}
void debug_out() { cerr << endl; }
template<typename Head, typename... Tail>
void debug_out(Head h, Tail... t) {
	cerr << " " << h;
	if (sizeof...(t) > 0) cerr << " :";
	debug_out(t...);
}
template <typename T>
ostream& operator<<(ostream& os, vector<T> vec) {
	for (size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
	return os;
}
ll ctoi(char c) {
	switch (c) {
	case '0': return 0; case '1': return 1; case '2': return 2;
	case '3': return 3; case '4': return 4; case '5': return 5;
	case '6': return 6; case '7': return 7; case '8': return 8;
	case '9': return 9;  default: return 0;
	}
}
bool pairCompare(const pll firstElof, pll secondElof)
{
	return firstElof.first > secondElof.first;
}
//**Snippetリスト**//
//rep, vin, all, iteLoop, bitSearch, bitList, nod//
//gcdlcm, isPrime, eratos, primeFactorize, Npow, combination, divisor, modinv, doubling, digitDP//
//dfs, bfs, dijkstra, WarshallFloyd, BellmanFord, UnionFind, Kruskal ,RMQ, LCA//
ll i, j, k, l; ll N, M, K, H, W, L, X, Y, Z, R, Q;
ll MOD = 1000000007, INF = 1LL << 60, ans = 0, z = 0, o = 1;
vl flag, D; vpll path;
//***********//
int main() {
	cin >> N;
	vl A(N);
	map<ll, ll> B;
	for (i = 0; i < N; i++) {
		cin >> A[i];
		B[A[i]]++;
	}
	sort(A.begin(), A.end());
	reverse(A.begin(), A.end());
	for (i = 0; i < N; i++) {
		if (B[A[i]] > 0) {
			B[A[i]]--;
			for (j = 31; j >= 1; j--) {
				ll P = pow(2, j) - A[i];
				if (P >= 1) {
					//debug(P, B[P]);
					if (B[P] > 0) {
						ans++;
						B[P]--;
						break;
					}
				}
			}
		}
	}
	cout << ans;
}
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0, i##_len = (n); i < i##_len; ++i)
using ll = long long;

int main() {
  ll n, a;
  cin >> n;
  multiset<ll, greater<>> st;
  rep(i, n) {
    cin >> a;
    st.insert(a);
  }
  ll ans = 0;
  while(!st.empty()) {
    auto it = st.begin();
    st.erase(it);
    ll p = 2;
    while(p <= *it) {
      p *= 2;
    }
    auto jt = st.find(p - *it);
    if(jt == st.end()) continue;
    st.erase(jt);
    ans++;
  }
  cout << ans << endl;
}