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

#define rep(i,n) for(long long i = 0; i < n; i++)
#define REP(i,s,n) for(long long i = (s); i <= (n); i++)
#define repr(i,n) for(long long i = n - 1; i >= 0; i--)
#define REPR(i,s,n) for(long long i = (s); i >= (n); i--)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define sumvec(v) accumulate(all(v), 0LL)
#define pb push_back
#define DOUBLE fixed << setprecision(15)
#define OK cerr << "OK\n"
#define OK1 cerr << "OK1\n"
#define OK2 cerr << "OK2\n"
#define sz(s) (long long)s.size()
#define INF ((1LL << 62) - (1LL << 31))
#define zero(x, n) setw(x) << setfill('0') << n
//#define endl '\n'

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<long long> vll;
typedef vector<double> vd;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<vd> vvd;
typedef vector<vc> vvc;
typedef vector<vb> vvb;
typedef pair<ll,ll> P;
typedef vector<P> vP;

template<class A, class B>
ostream& operator<<(ostream& ost, const pair<A, B>&p) {
    ost << "{" << p.first << ", " << p.second << "} ";
    return ost;
}

template<class T>
ostream& operator<<(ostream& ost, const vector<T>&v) {
    ost << "{";
    for (int i = 0; i<(int)v.size(); i++) {
        if (i)ost << " ";
        ost << v[i];
    }
    ost << "} \n";
    return ost;
}

template<class A, class B>
ostream& operator<<(ostream& ost, const map<A, B>&v) {
    ost << "{";
    for (auto p:v) {
        ost << "{" << p.first << ", " << p.second << "} ";
    }
    ost << "} ";
    return ost;
}

void out(){cout << "\n";}
template<class T,class... Args>
void out(const T &x,const Args &... args){
    cout << x << " ";
    out(args...);
}

template<typename T>
istream& operator >> (istream& is, vector<T>& vec){
    for(T& e: vec) is >> e;
    return is;
}

template<typename T>
string join(vector<T>& vec, string sep=" "){
    stringstream ss;
    for(int i=0;i<(int)vec.size();i++){
        ss << vec[i] << (i+1==(int)vec.size()?"\n":sep);
    }
    return ss.str();
}

template<class T>
inline bool chmax(T& a, T b){if(a<b){a=b;return true;} return false;}

template<class T>
inline bool chmin(T& a, T b){if(a>b){a=b;return true;} return false;}

#define INT(...) int __VA_ARGS__;IN(__VA_ARGS__)
#define LL(...) long long __VA_ARGS__;IN(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;IN(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;IN(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;IN(__VA_ARGS__)
#define LD(...) long double __VA_ARGS__;IN(__VA_ARGS__)

void scan(int& a){ cin >> a; }
void scan(long long& a){ cin >> a; }
void scan(char& a){ cin >> a; }
void scan(double& a){ cin >> a; }
void scan(string& a){ cin >> a; }
void scan(long double& a){ cin >> a; }

void IN(){}
template<class Head,class... Tail> void IN(Head& head,Tail&... tail){ scan(head); IN(tail...); }

void YES(bool b) {cout << ((b)?"YES\n":"NO\n");}
void Yes(bool b) {cout << ((b)?"Yes\n":"No\n");}
void yes(bool b) {cout << ((b)?"yes\n":"no\n");}
void Yay(bool b) {cout << ((b)?"Yay!\n":":(\n");}

const long double pi = acos(-1.0);
const ll mod=1000000007;
//const ll mod=998244353;

ll powmod(ll a,ll b){ll c=1; while(b>0){ if(b&1){ c=a*c%mod; } a=a*a%mod; b>>=1; } return c;}
ll nCrmod(ll n,ll r){ll x=1,y=1; for(ll i=0;i<r;i++) {x=x*(n-i)%mod; y=y*(i+1)%mod; } return x*powmod(y,mod-2)%mod;}
ll gcd(ll a,ll b){ while(b){ ll c=b; b=a%b; a=c; } return a; }
ll lcm(ll a,ll b){ if(!a || !b) return 0; return a*b/gcd(a,b); }
vll divisor(ll x){ vll v; for(int i=1;i*i<=x;i++)if(x%i==0){ v.pb(i);if(i*i!=x)v.pb(x/i); } sort(all(v)); return v; };
map<ll,ll> prime_factor(ll n){ map<ll,ll> m; for(ll i=2;i*i<=n;i++){ while(n%i==0){ m[i]++; n/=i; } } if(n!=1) m[n]=1; return m; }

int main()
{
    LL(n);
    vll a(n);
    cin >> a;
    ll ans = 0;
    multiset<ll> s(all(a));
    while(!s.empty()) {
        auto it = prev(s.end());
        ll a = *it;
        s.erase(it);
        ll b = (1LL << (64 - __builtin_clzll(a))) - a;
        it = s.find(b);
        if(it != s.end()) {
            ans++;
            s.erase(it);
        }
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 14060