/* Aa^~ kokoro ga pyompyon suru n jaa^~
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*/

#include <bits/stdc++.h>
using namespace std;

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)



const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;

int main(){
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  int N;
  cin >> N;
  VI xs(N);
  cin >> xs;

  map<int,int> cnt;
  for(int x: xs) ++cnt[x];

  LL MX = 2e9;
  map<int,VI> G;
  map<int,int> in;
  for(auto& p: cnt){
	LL p2 = 1;
	for(;p2<=MX;p2<<=1){
	  LL r = p2 - p.FF;
	  if(r > 0 && cnt.count(r)){
		G[p.FF].PB(r);
		in[r]++;
	  }
	}
  }

  queue<int> q;
  for(auto& p: in){
	if(p.SS == 1){
	  q.push(p.FF);
	}
  }

  int ans = 0;
  while(!q.empty()){
	int u = q.front();
	q.pop();
	int& w = cnt[u];
	for(auto to: G[u]){
	  int rw = min(w, u != to ? cnt[to] : cnt[to] / 2);
	  w -= rw;
	  cnt[to] -= rw;
	  ans += rw;

	  --in[to];
	  if(in[to] == 1){
		q.push(to);
	  }
	}
  }
  cout << ans << endl;
  

  return 0;
}


Measured memory_bytes: 37376