//17571@nith.ac.in @Ankit Verma
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;

#pragma GCC optimize("O3")
#define ll long long
#define ld long double


//-------------------------------------------------------------------------------------------------------------------------------------------------
// Custom Hash Function for Unordered Maps

struct custom_hash {
	static uint64_t splitmix64(uint64_t x) {
		// http://xorshift.di.unimi.it/splitmix64.c
		x += 0x9e3779b97f4a7c15;
		x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
		x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
		return x ^ (x >> 31);
	}

	size_t operator()(uint64_t x) const {
		static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
		return splitmix64(x + FIXED_RANDOM);
	}
};

//-------------------------------------------------------------------------------------------------------------------------------------------------

// Debugger Tools
#ifndef ONLINE_JUDGE
#define debug(...) __f(#__VA_ARGS__, __VA_ARGS__)
template < typename Arg1 >
void __f(const char* name, Arg1&& arg1) {
	cerr << name << " : " << arg1 << std :: endl;
}
template < typename Arg1, typename... Args >
void __f(const char* names, Arg1&& arg1, Args&&... args) {
	const char* comma = strchr(names + 1, ',');
	cerr.write(names, comma - names) << " : " << arg1 << " | ";
	__f(comma + 1, args...);
}
#else
#define debug(...)
#endif
#define n_l '\n'
template <typename T, size_t N> int SIZE(const T (&t)[N]) { return N; } template<typename T> int SIZE(const T &t) { return t.size(); } string to_string(string s, int x1 = 0, int x2 = 1e9) { return '"' + ((x1 < s.size()) ? s.substr(x1, x2 - x1 + 1) : "") + '"'; } string to_string(const char* s) { return to_string((string) s); } string to_string(bool b) { return (b ? "true" : "false"); } string to_string(char c) { return string({c}); } template<size_t N> string to_string(bitset<N> &b, int x1 = 0, int x2 = 1e9) { string t = ""; for (int __iii__ = min(x1, SIZE(b)),  __jjj__ = min(x2, SIZE(b) - 1); __iii__ <= __jjj__; ++__iii__) { t += b[__iii__] + '0'; } return '"' + t + '"'; } template <typename A, typename... C> string to_string(A (&v), int x1 = 0, int x2 = 1e9, C... coords); int l_v_l_v_l = 0, t_a_b_s = 0; template <typename A, typename B> string to_string(pair<A, B> &p) { l_v_l_v_l++; string res = "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; l_v_l_v_l--; return res; } template <typename A, typename... C> string to_string(A (&v), int x1, int x2, C... coords) { int rnk = rank<A>::value; string tab(t_a_b_s, ' '); string res = ""; bool first = true; if (l_v_l_v_l == 0) res += n_l; res += tab + "["; x1 = min(x1, SIZE(v)), x2 = min(x2, SIZE(v)); auto l = begin(v); advance(l, x1); auto r = l; advance(r, (x2 - x1) + (x2 < SIZE(v))); for (auto e = l; e != r; e = next(e)) { if (!first) { res += ", "; } first = false; l_v_l_v_l++; if (e != l) { if (rnk > 1) { res += n_l; t_a_b_s = l_v_l_v_l; }; } else { t_a_b_s = 0; } res += to_string(*e, coords...); l_v_l_v_l--; } res += "]"; if (l_v_l_v_l == 0) res += n_l; return res; } void dbgs() {;} template<typename Heads, typename... Tails> void dbgs(Heads H, Tails... T) { cout << to_string(H) << " | "; dbgs(T...); }
#define dbgm(...) cout << "[" << #__VA_ARGS__ << "]: "; dbgs(__VA_ARGS__); cout << endl;

// Mathematical Constants
const ll MOD              =        1000000007;
const ll MOD9             =        1000000009;
const double pie          =        2 * acos(0.0);
const int C_SIZE          =        121000;
const long long inf       =        MOD * MOD;

//-------------------------------------------------------------------------------------------------------------------------------------------------
// Utility Functions

#define eps                        1e-8
#define ull                        unsigned long long
#define eof                        (scanf("%d",&n))!= EOF
#define bolt                       ios::sync_with_stdio(false)
#define prec(n)                    fixed<<setprecision(n)
#define ms(s, n)                   memset(s, n, sizeof(s))
#define Unique(v)                  v.erase(unique(v.begin(), v.end()), v.end());
#define EraseSpace(s)              s.erase( remove( s.begin(), s.end(), ' ' ), s.end() );
#define leftrotate(str, n)         rotate(str.begin(), str.begin()+n, str.end());
#define rightrotate(str, n)        rotate(str.begin(), str.begin()+str.size()-n, str.end());
//-------------------------------------------------------------------------------------------------------------------------------------------------
// Constants and Shortcut Functions

#define pb                         push_back
#define eb                         emplace_back
#define ff                         first
#define ss                         second
#define mp                         make_pair
#define pii                        pair <int, int>
#define pll                        pair<ll, ll>
#define vll                        vector<ll>
#define vi                         vector<int>
#define mii                        map<int,int>
#define pqb                        priority_queue<ll>
#define pqs                        priority_queue<ll,vi,greater<ll>>
#define all(v)                     v.begin(), v.end()
#define setbits(x)                 __builtin_popcountll(x)
#define zrobits(x)                 __builtin_ctzll(x)
constexpr int dx[] =               { -1, 0, 1, 0, 1, 1, -1, -1};
constexpr int dy[] =               {0, -1, 0, 1, 1, -1, 1, -1};
#define f(i, x, n)                 for(int i = x; i < n; i++)
#define fr(i, n, x)                for(int i = n; i >= x; i--)

ll fact[C_SIZE];
ll finv[C_SIZE];
ll inv[C_SIZE];

ll Comb(int a, int b) {
	if (a < b || b < 0)return 0;
	return fact[a] * finv[b] % MOD * finv[a - b] % MOD;
}

gp_hash_table<ll, ll, custom_hash> smp;
typedef tree <ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> pbds;

void c_p_c()
{
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
}

void run_case() {

	int n; cin >> n;
	multiset <ll> mtl;
	for (int i = 0; i < n; i++) {
		ll x; cin >> x;
		mtl.insert(x);
	}
	int res = 0;
	while (!mtl.empty()) {
		auto ed = *mtl.rbegin();
		mtl.erase(mtl.find(ed));
		ll step = pow(2, logb(ed) + 1);
		auto it = mtl.find(step - ed);
		if (it != mtl.end()) {
			mtl.erase(it);
			res++;
		}
	}
	cout << res << endl;

}

int main() {

	//c_p_c();
	bolt;
	cin.tie(nullptr);
	cout.tie(nullptr);

	int tests = 1; //cin >> tests;

	while (tests-- > 0) {
		run_case();
	}

	return 0;
}

Measured memory_bytes: 9600