#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <string.h>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i,n) for(int i=0;i<(lint)(n);i++)
#define REP(i,n) for(int i=1;i<=(lint)(n);i++)
#define all(V) V.begin(),V.end()
#define stackReplaceBegin(size)\
	void* p=malloc(size);\
	esp_new=(lint)p+size-1;\
	void * stack_extend_memory_ = malloc(size);\
	void * stack_extend_origin_memory_;\
	char * stack_extend_dummy_memory_ = (char*)alloca((1+(int)(((long long)stack_extend_memory_)&127))*16);\
	*stack_extend_dummy_memory_ = 0;\
	asm volatile("mov %%rsp, %%rbx\nmov %%rax, %%rsp":"=b"(stack_extend_origin_memory_):"a"((char*)stack_extend_memory_+(size)-1024));
typedef long long lint;
typedef unsigned long long ulint;
typedef std::pair<int, int> P;
constexpr int INF = INT_MAX/2;
constexpr lint LINF = LLONG_MAX/2;
constexpr double eps = DBL_EPSILON;
constexpr double PI = 3.141592653589793238462643383279;
lint esp_new;
template<class T>
class prique :public std::priority_queue<T, std::vector<T>, std::greater<T>> {};
template <class T, class U>
inline bool chmax(T& lhs, const U& rhs) {
	if (lhs < rhs) {
		lhs = rhs;
		return 1;
	}
	return 0;
}
template <class T, class U>
inline bool chmin(T& lhs, const U& rhs) {
	if (lhs > rhs) {
		lhs = rhs;
		return 1;
	}
	return 0;
}
inline lint gcd(lint a, lint b) {
	while (b) {
		lint c = a;
		a = b; b = c % b;
	}
	return a;
}
inline lint lcm(lint a, lint b) {
	return a / gcd(a, b) * b;
}
bool isprime(lint n) {
	if (n == 1)return false;
	for (int i = 2; i * i <= n; i++) {
		if (n % i == 0)return false;
	}
	return true;
}
template<typename T>
T mypow(T a, unsigned int b) {
	T res(1),now(a);
	while(b){
		if(b&1)res*=now;
		b>>=1;
		now*=now;
	}
	return res;
}
lint modpow(lint a,unsigned int b,unsigned int mod){
	lint res(1),now(a%mod);
	while(b){
		if(b&1){
			res*=now;
			res%=mod;
		}
		b>>=1;
		now*=now;
		now%=mod;
	}
	return res;
}
template<typename T>
void printArray(std::vector<T>& vec) {
	rep(i, (int)vec.size() - 1)std::cout << vec[i] << " ";
	if(!vec.empty())std::cout << vec.back() << std::endl;
}
template<typename T>
void printArray(T l, T r) {
	T rprev = r;
	rprev--;
	for (T i = l; i != rprev; i++) {
		std::cout << *i << " ";
	}
	std::cout << *rprev << std::endl;
}
int n,a[200010];
std::map<int,int> mp;
std::vector<int> vec;
int main(){
	std::cin>>n;
	rep(i,n){
		std::cin>>a[i];
		mp[a[i]]++;
		vec.emplace_back(a[i]);
	}
	std::sort(all(vec));
	vec.erase(std::unique(all(vec)),vec.end());
	int ans=0;
	for(int i=vec.size()-1;i>=0;i--){
		int cnt=1;
		while(cnt<=vec[i])cnt*=2;
		int d=std::min(mp[vec[i]],mp[cnt-vec[i]]);
		if(vec[i]==cnt-vec[i])d=mp[vec[i]]/2;
		ans+=d;
		mp[vec[i]]-=d;
		mp[cnt-vec[i]]-=d;
	}
	std::cout<<ans<<std::endl;
	return 0;
}

Measured memory_bytes: 23468