#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#  include <intrin.h>
#  define __builtin_popcount __popcnt
#endif

#define _USE_MATH_DEFINES

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <unordered_map>
#include <stack>
#include <list>
#include <math.h>
#include <bitset>
#include <iterator>
#include <map>
#include <iomanip>
#include <functional>
#include <string>
#include <algorithm>
#include <queue>
#include <random>
#include <numeric>
#include <set>
#include <cassert>
#include <cmath>
#include <cinttypes>

using namespace std;

template<class T> ostream& operator<<(ostream& os, const vector<T>& v) { for (auto i = begin(v); i != end(v); i++) os << *i << (i == end(v) - 1 ? "" : " "); return os; }
template<class T> istream& operator>>(istream& is, vector<T>& v) { for (auto i = begin(v); i != end(v); i++) is >> *i; return is; }
struct fast_io {
	fast_io& operator>> (int& target) { scanf("%" PRId32, &target);  return *this; }
	fast_io& operator>> (unsigned int& target) { scanf("%" PRIu32, &target);  return *this; }
	fast_io& operator>> (long long& target) { scanf("%" PRId64, &target);  return *this; }
	fast_io& operator>> (unsigned long long& target) { scanf("%" PRIu64, &target);  return *this; }
	template<typename T> fast_io& operator>> (vector<T>& target) { for (auto& value : target) { *this >> value; } return *this; }
	fast_io& operator<< (const int value) { printf("%" PRId32, value); return *this; }
	fast_io& operator<< (const unsigned int value) { printf("%" PRIu32, value); return *this; }
	fast_io& operator<< (const long long value) { printf("%" PRId64, value); return *this; }
	fast_io& operator<< (const unsigned long long value) { printf("%" PRIu64, value); return *this; }
	fast_io& operator<< (const string& value) { printf("%s", value.c_str()); return *this; }
	template<typename T> fast_io& operator<< (const vector<T>& target) { for (auto& value : target) { *this << value << " "; } return *this; }
}; fast_io fio;

template<class ForwardIt, class UnaryPredicate>
ForwardIt first_true(ForwardIt first, ForwardIt last, UnaryPredicate predicate) {
	ForwardIt it;
	typename std::iterator_traits<ForwardIt>::difference_type count, step; count = std::distance(first, last);
	while (count > 0) {
		it = first;	step = count / 2;
		std::advance(it, step);
		if (!predicate(*it)) { first = ++it; count -= step + 1; }
		else count = step;
	}
	return first;
}

#define MOD 1000000007
#define pii pair<int,int>
#define pll pair<long long, long long>
#define all(x) (x).begin(), (x).end()
#define sort_all(x) sort(all(x))
#define make_unique(x) (x).erase(unique(all(x)), (x).end())



int main() {

#if defined(_DEBUG)
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif

	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);

	int n;
	fio >> n;
	vector<int> arr(n);
	fio >> arr;

	map<int, int> mm;
	for (int x : arr) mm[x]++;

	int ans = 0;
	for (auto it = mm.rbegin(); it != mm.rend(); it++)
	{
		int val = it->first;
		int j = 1;
		while (j <= val) j *= 2;

		while (mm[val] > 0)
		{
			auto it2 = mm.find(j - it->first);
			if (it2 == mm.end()) break;
			if ((j-val) == val && it->second > 1)
			{
				mm[val] -= 2;
				ans++;
			}
			else if ((j-val) != val && it2->second > 0)
			{
				mm[val] -= 1;
				mm[j - it->first]--;
				ans++;
			}
			else break;
		}


	}

	fio << ans;

	return 0;
}

Measured memory_bytes: 10368