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

#define MP make_pair
#define PB push_back
#define ALL(x)      (x).begin(),(x).end()
#define REP(i,n)    for(int i=0;i<(n);i++)
#define REP1(i,n)   for(int i=1;i<(n);i++)
#define REP2(i,d,n) for(int i=(d);i<(n);i++)
#define RREP(i,n)   for(int i=(n);i>=0;i--)
#define CLR(a)      memset((a),0,sizeof(a))
#define MCLR(a)     memset((a),-1,sizeof(a))
#define RANGE(x,y,maxX,maxY) (0 <= (x) && 0 <= (y) && (x) < (maxX) && (y) < (maxY))

typedef long long LL;
typedef unsigned long long ULL;
typedef vector<int> VI;
typedef vector<VI > VVI;
typedef vector<string> VS;
typedef vector<LL> VLL;
typedef pair<int,int> PII;

const int INF = 0x3f3f3f3f;
const LL INFL = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-9;

const int DX[]={1,0,-1,0},DY[]={0,-1,0,1};

void solve(long long N, std::vector<long long> A){
    map<int, int> memo;
    REP(i, N) {
        if(memo.count(A[i]) == 0) {
            memo[A[i]] = 0; 
        }

        memo[A[i]]++;
    }

    VLL beki;
    LL idx = 1;
    while(idx <= 2000000000LL) {
        beki.PB(idx);
        idx *= 2LL;
    }

    sort(ALL(A));
    A.erase(unique(ALL(A)), A.end());
    int ans = 0;
    RREP(i, A.size()-1) {
        REP(j, beki.size()) {
            int idx = beki[j] - A[i];
            if(memo.count(idx) == 1) {
                if(A[i] == idx) {
                    int add_ans = memo[idx] / 2;
                    memo[idx] -= add_ans * 2;
                    ans += add_ans;
                } else {
                    int add_ans = min(memo[idx], memo[A[i]]);
                    memo[idx] -= add_ans;
                    memo[A[i]] -= add_ans;
                    ans += add_ans;
                }
            }
        }
    }

    cout << ans << endl;
}

int main(){
    long long N;
    scanf("%lld",&N);
    std::vector<long long> A(N);
    for(int i = 0 ; i < N ; i++){
        scanf("%lld",&A[i]);
    }
    solve(N, std::move(A));
    return 0;
}


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

template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

#define rep(i,n) for (int i = 0; i < n; ++i)
#define reps(i,s,n) for (int i = s; i < n; ++i)
#define rep1(i,n) for (int i = 1; i <= n; ++i)
#define per(i,n) for (int i = n - 1; i >= 0; --i)
#define per1(i,n) for (int i = n; i >= 1; --i)
#define all(c) begin(c),end(c)
template<typename T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<typename T> inline T intceil(T a, T b) { return (a + (b - 1)) / b; }
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;
typedef tuple<int,int,int> Tint;
typedef pair<ll,ll> PL;
const long long MOD = 1e9+7;
#define precout() cout << std::fixed << std::setprecision(20);
#define print(a) cout << a << endl;
template<typename T> void printv(vector<T>&v) {
    int n = v.size() - 1;
    rep(i, n) cout << v[i] << " ";
    cout << v.back() << endl;
}
const string alphabet = "abcdefghijklmnopqrstuvwxyz";
const int dy[4] = { 0, 1, 0, -1 };
const int dx[4] = { 1, 0, -1, 0 };
// if(nextH >= 0 && nextH < H && nextW >= 0 && nextW < W)
static const long double pi = acos(-1.0);
typedef complex<ld> cd;

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

    int N; cin >> N;
    vector<ll> A(N);
    unordered_map<ll, int> cnt;
    rep(i, N) {
        cin >> A[i];
        ++cnt[A[i]];
    }
    sort(all(A), greater<ll>());

    int ans = 0;
    rep(i, N) {
        ll cur = A[i];
        if(cnt[cur] == 0) continue;

        ll power = 2;
        while(power <= cur) {
            power *= 2;
        }
        ll need = power - cur;
        if(cnt[need] > (need == cur ? 1 : 0)) {
            // debug(cur, need);
            ++ans;
            --cnt[cur];
            --cnt[need];
        }
    }

    print(ans);
}
