#include <iostream>
#include <iomanip>
#include <string>
#include <stack>
#include <vector>
#include <complex>
#include <math.h>
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <iterator>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <regex>
#include <limits>
#include <time.h>
#include <cstdint>
using namespace std;
using pii  = pair<int,int>;
using ll=long long;
using ld=long double;
#define pb push_back
#define mp make_pair
#define sc second
#define fr first
#define stpr setprecision
#define cYES cout<<"YES"<<endl
#define cNO cout<<"NO"<<endl
#define cYes cout<<"Yes"<<endl
#define cNo cout<<"No"<<endl
#define rep(i,n) for(ll i=0;i<(n);++i)
#define Rep(i,a,b) for(ll i=(a);i<(b);++i)
#define rrep(i,n) for(ll i=n-1;i>=0;i--)
#define rRep(i,a,b) for(ll i=a;i>=b;i--)
#define crep(i) for(char i='a';i<='z';++i)
#define psortsecond(A,N) sort(A,A+N,[](const pii &a, const pii &b){return a.second<b.second;});
#define ALL(x) (x).begin(),(x).end()
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define endl '\n'
int ctoi(const char c){
  if('0' <= c && c <= '9') return (c-'0');
  return -1;
}
ll gcd(ll a,ll b){return (b == 0 ? a : gcd(b, a%b));}
ll lcm(ll a,ll b){return a*b/gcd(a,b);}
constexpr ll MOD=1000000007;
constexpr ll INF=1000000011;
constexpr ll MOD2=998244353;
constexpr ll LINF = 1001002003004005006ll;
constexpr ld EPS=10e-8;
template <class T, class U> inline bool chmax(T& lhs, const U& rhs) { if (lhs < rhs) { lhs = rhs; return 1; } return 0; }
template <class T, class U> inline bool chmin(T& lhs, const U& rhs) { if (lhs > rhs) { lhs = rhs; return 1; } return 0; }
template<typename T> istream& operator>>(istream& is,vector<T>& v){for(auto&& x:v)is >> x;return is;}
template<typename T,typename U> istream& operator>>(istream& is, pair<T,U>& p){ is >> p.first; is >> p.second; return is;}
template<typename T,typename U> ostream& operator>>(ostream& os, const pair<T,U>& p){ os << p.first << ' ' << p.second; return os;}
template<class T> ostream& operator<<(ostream& os, vector<T>& v){
  for(auto i=begin(v); i != end(v); ++i){
    if(i !=begin(v)) os << ' ';
    os << *i;
  }
  return os;
}

ll A[200007];
map<ll,ll> M;

int main(){
  ll N;
  cin >> N;
  rep(i,N){
    cin >> A[i];
    M[A[i]]++;
  }
  sort(A,A+N);
  reverse(A,A+N);
  ll ANS=0;
  rep(i,N){
    rrep(k,31){
      ll j=1<<k;
      if(A[i]>j){
        break;
      }
      if(M.find(j-A[i])!=M.end()){
        if(((j-A[i]!=A[i] && M[j-A[i]]>0) && M[A[i]]>0  )|| (j-A[i]==A[i] &&  M[j-A[i]]>1)){
          M[A[i]]--;
          M[j-A[i]]--;
          ANS++;
          break;
        }
      }
    }
  }
  cout << ANS << endl;
}

#include <bits/stdc++.h>
#define INF 100000000000
#define ll long long
#define pll pair<ll, ll>
using namespace std;

int main()
{
  ll N;
  cin >> N;
  vector<ll> A(N);
  for (ll i = 0; i < N; ++i) {
    cin >> A.at(i);
  }
  sort(A.begin(), A.end());
  ll ans = 0;
  vector<bool> used(N, false);
  for (ll i = N - 1; i >= 0; --i) {
    if (used.at(i)) {
      continue;
    }
    used.at(i) = true;
    ll pow2 = 1;
    while (pow2 <= A.at(i)) {
      pow2 *= 2;
    }
    ll need = pow2 - A.at(i);
    ll l = lower_bound(A.begin(), A.end(), need) - A.begin() - 1;
    ll r = upper_bound(A.begin(), A.end(), need) - A.begin();
    r = min(r, i);
    // cout << "nned" << need << endl;
    // cout << l << ' ' << r << endl;
    if (l == r) {
      continue;
    }
    while (l + 1 != r) {
      ll m = (l + r) / 2;
      if (used.at(m)) {
        l = m;
      }
      else {
        r = m;
      }
    }
    // cout << "l" << l << "r" << r << endl;
    if (r == N) {
      continue;
    }
    if (A.at(r) != need) {
      continue;
    }
    if (used.at(r)) {
      continue;
    }
    used.at(r) = true;
    ans += 1;
    // for (ll j = l; j < r; ++j) {
    //   if (!used.at(j)) {
    //     used.at(j) = true;
    //     ans += 1;
    //     break;
    //   }
    // }
  }
  cout << ans << endl;
}
