//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~


int main() {

    int N;
    cin >> N;

    vector<LL> A(N);
    for (int i = 0; i < N; i++) cin >> A[i];
    sort(A.rbegin(), A.rend());

    multiset<LL> st;
    for (int i = 0; i < N; i++) st.insert(A[i]);


    vector<LL> powers;
    for (LL i = 0; i < 40; i++) powers.push_back(1LL << i);

    int ans = 0;
    for (int i = 0; i < N; i++) {
        if (st.size() == 0) break;
        if (st.find(A[i]) != st.end()) {
            st.erase(st.find(A[i]));
            for (int j = 0; j < 40; j++) {
                LL b = powers[j] - A[i];
                if (b >= 1) {
                    if (st.find(b) != st.end()) {
                        ans++;
                        st.erase(st.find(b));
                    }
                }
            }
        }
    }

    cout << ans << endl;

    return 0;
}







































Measured memory_bytes: 11136