//int nxt(int x){
//	x += 1;
//	while(__builtin_popcount(x) > 1)
//		x += x & (-x);
//	return x;
//}
//void _(){
//	int n;
//	cin >> n;
//	vi arr(n);
//	cin >> arr;
//	multiset<int> s(all(arr));
//	int ans = 0;
//	while(s.size() >= 2){
//		int x = *--s.end();
//		s.erase(--s.end());
//		int y = nxt(x)-x;
//		auto it = s.find(y);
//		if(it != s.end()){
//			++ans;
//			s.erase(it);
//		}
//	}
//	print(ans);
//}
//
//
// ================================================ 
// ^^^	ABOVE is what I wrote in my editor	^^^
// ------------------------------------------------ 
// vvv	BELOW is automatically generated 	vvv
// ================================================ 
#include <set>
#include <iomanip>
#include <cassert>
#include <vector>
#include <iostream>
#include <string>
#define all(v) (v).begin(),(v).end()
#define sz(v) ll(v.size())
using namespace std;
typedef long long ll;
typedef vector<int> vi;
template<typename IT>
ostream &print(IT b, IT e, string sep = " ", ostream &stream = cout){
    for(IT it = b; it != e; ++it){
        stream << *it;
        if(it+1 != e) stream << sep;
    } 
    return stream;
}
template<typename IT> istream &read(IT b, IT e, istream &stream = cin){
    for(IT it = b; it != e; ++it)
        stream >> *it; 
    return stream;
}
template<typename T> ostream &operator<<(ostream &stream, vector<T> &t){
    return print(all(t)," ",stream);
}
template<typename T> ostream &operator<<(ostream &stream, vector<T> &&t){
    return print(all(t)," ",stream);
}
template<typename T> istream &operator>>(istream &stream, vector<T> &t){
    return read(all(t),stream);
}
template <typename T> 
void print(T x, string end = "\n"){
    cout << x << end;
}
template <typename t> 
void print(vector<t> arr,string sep=" "){
    if(arr.empty()){
        return;
    }
    print(arr.begin(),arr.end(),sep);
    cout << '\n';
}
template <typename T> 
void print(vector<vector<T> > arr){
    for(int i = 0; i < sz(arr); ++i){
        cout << "[" << arr[i] << "]";
        if(i+1 < sz(arr))
            cout << ", ";
    }
    cout << '\n';
}
template<typename T>
T _();
const ll MOD = 1e9+7;
template<typename T>
vector<T> operator+(const vector<T> &lhs, const vector<T> &rhs){
    vector<T> ans(lhs);
    assert(ans.size() == rhs.size());
    for(int i = 0; i < rhs.size(); ++i)
        ans[i] += rhs[i];
    return ans;
}
template<typename T>
vector<T> operator+=(vector<T> &lhs, const vector<T> &rhs){
    return lhs = lhs+rhs;
}
template<typename T>
vector<T> operator-(const vector<T> lhs, const vector<T> rhs){
    vector<T> ans(all(lhs));
    assert(ans.size() == rhs.size());
    for(int i = 0; i < rhs.size(); ++i)
        ans[i] -= rhs[i];
    return ans;
}
template<typename T>
vector<T> operator-=(vector<T> &lhs, const vector<T> &rhs){
    return lhs = lhs-rhs;
}
template<typename T>
vector<T> append(vector<T> &lhs, const vector<T> rhs){
    lhs.insert(lhs.end(),all(rhs));
    return lhs;
}
template<typename T>
T max(const vector<T> arr){
    assert(!arr.empty());
    T ans = arr[0];
    for(auto &cur : arr)
        ans = max(ans,cur);
    return ans;
}
template<typename T>
T max(const set<T> s){
    assert(!s.empty());
    return *--s.end();
}
template<typename T, typename G>
vector<T> set_max(vector<T> &base, const vector<G> &comp){
    assert(base.size() == comp.size());
    for(int i = 0; i < (int)comp.size(); ++i)
        base[i] = max(base[i],comp[i]);
    return base;
}
int nxt(int x){
	x += 1;
	while(__builtin_popcount(x) > 1)
		x += x & (-x);
	return x;
}
void _(){
	int n;
	cin >> n;
	vi arr(n);
	cin >> arr;
	multiset<int> s(all(arr));
	int ans = 0;
	while(s.size() >= 2){
		int x = *--s.end();
		s.erase(--s.end());
		int y = nxt(x)-x;
		auto it = s.find(y);
		if(it != s.end()){
			++ans;
			s.erase(it);
		}
	}
	print(ans);
}

int main(){
    ios_base::sync_with_stdio(false); cin.tie(0);
    cout << fixed << setprecision(15);
    _();
}


Measured memory_bytes: 10368