import java.util.*; import java.io.*; import java.math.*;
public class Main{
	//Don't have to see. start------------------------------------------
	static class InputIterator{
		ArrayList<String> inputLine = new ArrayList<String>(1024);
		int index = 0; int max; String read;
		InputIterator(){
			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
			try{
				while((read = br.readLine()) != null){
					inputLine.add(read);
				}
			}catch(IOException e){}
			max = inputLine.size();
		}
		boolean hasNext(){return (index < max);}
		String next(){
			if(hasNext()){
				return inputLine.get(index++);
			}else{
				throw new IndexOutOfBoundsException("There is no more input");
			}
		}
	}
	static HashMap<Integer, String> CONVSTR = new HashMap<Integer, String>();
	static InputIterator ii = new InputIterator();//This class cannot be used in reactive problem.
	static PrintWriter out = new PrintWriter(System.out);
	static void flush(){out.flush();}
	static void myout(Object t){out.println(t);}
	static void myerr(Object t){System.err.print("debug:");System.err.println(t);}
	static String next(){return ii.next();}
	static boolean hasNext(){return ii.hasNext();}
	static int nextInt(){return Integer.parseInt(next());}
	static long nextLong(){return Long.parseLong(next());}
	static double nextDouble(){return Double.parseDouble(next());}
	static ArrayList<String> nextStrArray(){return myconv(next(), 8);}
	static ArrayList<String> nextCharArray(){return myconv(next(), 0);}
	static ArrayList<Integer> nextIntArray(){
		ArrayList<String> input = nextStrArray(); ArrayList<Integer> ret = new ArrayList<Integer>(input.size());
		for(int i = 0; i < input.size(); i++){
			ret.add(Integer.parseInt(input.get(i)));
		}
		return ret;
	}
	static ArrayList<Long> nextLongArray(){
		ArrayList<String> input = nextStrArray(); ArrayList<Long> ret = new ArrayList<Long>(input.size());
		for(int i = 0; i < input.size(); i++){
			ret.add(Long.parseLong(input.get(i)));
		}
		return ret;
	}
	static String myconv(Object list, int no){//only join
		String joinString = CONVSTR.get(no);
		if(list instanceof String[]){
			return String.join(joinString, (String[])list);
		}else if(list instanceof ArrayList){
			return String.join(joinString, (ArrayList)list);
		}else{
			throw new ClassCastException("Don't join");
		}
	}
	static ArrayList<String> myconv(String str, int no){//only split
		String splitString = CONVSTR.get(no);
		return new ArrayList<String>(Arrays.asList(str.split(splitString)));
	}
	public static void main(String[] args){
		CONVSTR.put(8, " "); CONVSTR.put(9, "\n"); CONVSTR.put(0, "");
		solve();flush();
	}
	//Don't have to see. end------------------------------------------
	static void solve(){//Here is the main function
      int N = nextInt();
      ArrayList<Integer> list = nextIntArray();
      TreeMap<Integer, Integer> map = new TreeMap<Integer, Integer>();
      for(int i = 0; i < N; i++){
        if(map.containsKey(list.get(i))){
          map.put(list.get(i), map.get(list.get(i)) + 1);
        }else{
          map.put(list.get(i), 1);
        }
      }
      Collections.sort(list, Comparator.reverseOrder());
      int count = 0;
      for(int i = 0; i < N; i++){
        int Ai = list.get(i);
        if(!map.containsKey(Ai)){
          continue;
        }
        int beki = 0;
        while(Ai >= Math.pow(2, beki)){
          beki++;
        }
        int check = (int)Math.pow(2, beki) - Ai;
        if(check == Ai){
          if(map.get(check) >= 2){
            map.put(check, map.get(check) - 2);
            count++;
            if(map.get(check) == 0){
              map.remove(check);
            }
          }
          continue;
        }
        if(map.containsKey(check)){
          map.put(check, map.get(check) - 1);
          map.put(Ai, map.get(Ai) - 1);
          count++;
          if(map.get(Ai) == 0){
            map.remove(Ai);
          }
          if(map.get(check) == 0){
            map.remove(check);
          }
        }
      }
      myout(count);
	}
	//Method addition frame start



	//Method addition frame end
}


Measured memory_bytes: 86356