#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("YES");}else{puts("NO");}
const int MX = 200005;

// 2分探索でペア探し
int search_pair(ll aa, int x, int y, ll a[], int flag[]){
    //再帰の終了条件
    if(x == y){
        if(flag[x] == 1){
            return (-1);
        }else{
            if(aa == a[x]){
                return x;
            }else{
                return (-1);
            }
        }
    }else{
        if(aa == a[(x+y)/2]){
            if(flag[(x+y)/2] == 0){
                return ((x+y)/2);
            }else{
                int new_pair = (search_pair(aa, (x+y)/2+1, y, a, flag));
                if(new_pair == -1){
                    new_pair = search_pair(aa, x, (x+y)/2, a, flag);
                }
                return new_pair;
            }
        }else{
            if(aa > a[(x+y)/2]){
                return (search_pair(aa, (x+y)/2+1, y, a, flag));
            }else{
                return (search_pair(aa, x, (x+y)/2, a, flag));
            }
        }
    }
}
 
int main() {
    int n;
    scanf("%d",&n);

    ll a[n] = {};
    int flag[n] = {};

    rep(i,n){
        scanf("%lld",&a[i]);
    }

    sort(a, a+n);

    int count = 0;

    for(int i = n-1; i>=1; --i){
        // スキップ条件
        if(flag[i] == 1){
            continue;
        }

        // (補数) = ((2のk乗) - a[i])作成
        ll hosuu = 1;
        while(hosuu <= a[i]){
            hosuu = hosuu * 2;
        }
        hosuu = hosuu - a[i];

        //ペア探し
        int pair = search_pair(hosuu, 0, i-1, a, flag);

        //事後処理
        if(pair == -1){
            ;
        }else{
            flag[pair] = 1;
            count++;
        }
    }

    int ans = 0;
    ans = count;
    cout << ans << endl;
   
    return 0;
}
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Measured memory_bytes: 2560