import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.HashMap;
import java.io.IOException;
import java.util.Comparator;
import java.util.NoSuchElementException;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FasterScanner in = new FasterScanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        public void solve(int testNumber, FasterScanner in, PrintWriter out) {
            int n = in.nextInt();
            ArrayList<Integer> a = new ArrayList<>();
            HashMap<Integer, Integer> map = new HashMap<>();

            for (int i = 0; i < n; i++) {
                int cn = in.nextInt();
                a.add(cn);
                map.put(cn, map.getOrDefault(cn, 0) + 1);
            }

            a.sort(Comparator.reverseOrder());

            int com = Integer.MAX_VALUE;

            int ans = 0;
            for (int i = 0; i < n; i++) {
                int cn = a.get(i);
                if (map.get(cn) <= 0) continue;

                // System.out.println(cn);
                while (cn <= (com >> 1)) {
                    com = com >> 1;
                }
                int target = 1 + (com ^ cn);
                //System.out.println(cn + " " + target + " " + com);
                if (cn == target) {
                    if (map.get(cn) >= 2) {
                        map.put(cn, map.get(cn) - 2);
                        ans++;
                    }
                    continue;
                } else if (map.getOrDefault(target, 0) > 0) {
                    ans++;
                    map.put(cn, map.get(cn) - 1);
                    map.put(target, map.get(target) - 1);
                }
            }

            out.println(ans);
        }

    }

    static class FasterScanner {
        private InputStream in;
        private byte[] buffer;
        private int bufPointer;
        private int bufLength;

        public FasterScanner(InputStream in) {
            this.in = in;
            buffer = new byte[1024];
        }

        private boolean hasNextByte() {
            if (bufPointer < bufLength) return true;
            bufPointer = 0;
            try {
                bufLength = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            return (bufLength > 0);
        }

        private int readByte() {
            if (hasNextByte()) return buffer[bufPointer++];
            return -1;
        }

        private static boolean isPrintableChar(int c) {
            return (c >= 33 && c <= 126);
        }

        public boolean hasNext() {
            while (hasNextByte() && !isPrintableChar(buffer[bufPointer])) bufPointer++;
            return hasNextByte();
        }

        public int nextInt() {
            long nl = nextLong();
            if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
            return (int) nl;
        }

        public long nextLong() {
            if (!hasNext()) throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || b > '9') throw new NumberFormatException();

            while (true) {
                if (b >= '0' && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b)) {
                    return minus ? -n : n;
                } else {
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }

    }
}



Measured memory_bytes: 60208