#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define ld long double
#define mod 998244353LL
#define eps 1e-13
// #define PI 3.141592653589793238L
#define INF 1000000011
#define INFLL 1000000000000000011LL
#define vi vector<int>
#define vll vector<long long>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define pil pair<int, long long>
#define pli pair<long long, int>
#define mp make_pair
#define F first
#define S second
#define pb push_back
#define fo(i,a,n) for(i = (a); i < (n); i++)
#define gtl(x) getline(cin, (x))
#define flsh fflush(stdout)
#define sws ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define gcd __gcd
#define all(a) (a).begin(), (a).end()
#define sz(a) (int)((a).size())
#define io_file freopen("D:/Coding Problems/Contest/input_file.in", "r", stdin); freopen("D:/Coding Problems/Contest/output_file.out", "w", stdout)

ll modx(ll Base, ll exponent)
{
	ll ans = 1;
	if(Base == 1)
		return Base;
	while(exponent)
	{
		if(exponent & 1)
			ans = (ans * Base)%mod;
		Base = (Base * Base)%mod;
		exponent = exponent >> 1;
	}
	return ans;
}

ll inmodx(ll num)
{
	return (modx(num, mod-2LL));
}

bool cmp(pair < pii, int > a, pair < pii, int > b)//true for a before b
{
	if(a.F.F/300 < b.F.F/300)
		return 1;
	if(a.F.F/300 > b.F.F/300)
		return 0;
	if(a.F.S < b.F.S)
		return 1;
	if(a.F.S > b.F.S)
		return 0;
	if(a.F <= b.F)
		return 1;
	return 0;
}

const int N = (2e5) + 9;
const int M = (N<<2) + 9;
const int LOGN = ((int)log2(N)) + 3;
const int LOGM = ((int)log2(M)) + 3;
const int BUCK = 2*550;
const int SQRT = BUCK+9;

const int alpha = 31;

ll a[N];
int cnt[N], ps_cnt[N];
map < ll, int > mapp;
vll arr;
vi v[N];
ll p2[N];
int deg[N];
set < int > temp;
queue < int > q;
bool mark[N];
priority_queue < pii > pq;

void perform(int n, int &ans)
{
	int i, val, z;
	fo(i,1,n+1)
		deg[i] = sz(v[i]);
	while(!pq.empty())
		pq.pop();
 
	fo(i,1,n+1)
		pq.push({-deg[i],i});
 
	while(!pq.empty())
	{
		z = pq.top().S;
		pq.pop();
		if(deg[z] <= 0)
			continue;
		deg[z] = 0;
		for(auto x : v[z])
			if(deg[x] > 0)
			{
				val = min(cnt[x],cnt[z]);
				ans += val;
				cnt[x] -= val;
				cnt[z] -= val;
				deg[x]--;
				if(deg[x] > 0)
					pq.push({-deg[x],x});
			}
	}
	return;
}

void solve()
{
	int n, i, j, ans = 0, z;
	p2[0] = 1;
	fo(i,1,alpha+1)
		p2[i] = p2[i-1]+p2[i-1];
	cin >> n;
	fo(i,1,n+1)
	{
		cin >> a[i];
		temp.insert(a[i]);
	}
	arr.pb(0);
	for(auto x : temp)
		arr.pb(x);
	fo(i,1,sz(arr))
		mapp[arr[i]] = i;
	fo(i,1,n+1)
		cnt[mapp[a[i]]]++;

	n = sz(arr)-1;
	fo(i,1,n+1)
		fo(j,0,alpha+1)
			if(p2[j]-arr[i] > arr[i])
			{
				z = mapp[p2[j]-arr[i]];
				if(z > 0)
				{
					v[i].pb(z);
					v[z].pb(i);
				}
			}
	fo(i,1,n+1)
		if((arr[i]&(arr[i]-1LL)) == 0LL)
		{
			ps_cnt[i] = cnt[i];
			cnt[i] = 0;
		}
	perform(n,ans);
	fo(i,1,n+1)
		if(ps_cnt[i] != 0)
			cnt[i] = ps_cnt[i];
	perform(n,ans);
	fo(i,1,n+1)
		if(cnt[i] > 0 && (arr[i]&(arr[i]-1LL)) == 0LL)
			ans += (cnt[i]/2);
	cout << ans << '\n';
	return;
}

int main()
{
	sws;
	clock_t clk;
	clk = clock();
	// io_file;
	// srand (time(NULL));

	//Code here
	int t = 1, cs;
	cout << fixed << setprecision(9);
	// cin >> t;
	fo(cs,1,t+1)
	{
		// cout << "Case #" << cs << ": ";
		solve();
	}
	// Code ends here

	clk = clock() - clk;
	cerr << fixed << setprecision(6) << "Time: " << ((double)clk)/CLOCKS_PER_SEC << "\n";
	return 0;
}

Measured memory_bytes: 78576