#pragma GCC optimize ("O3")
#include "bits/stdc++.h"

using namespace std;
using ll = long long int;

#define debugos cout
#define debug(v) {printf("L%d %s > ",__LINE__,#v);debugos<<(v)<<endl;}
#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:(v)){debugos<<e<<" ";}debugos<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){debugos<<(m)[x]<<" ";}debugos<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){debugos<<(m)[y][x]<<" ";}debugos<<endl;}}
#define ALL(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(remove_reference<remove_const<decltype(l)>::type>::type cnt=0;(cnt)<(l);++(cnt))
#define rrepeat(cnt,l) for(auto cnt=(l)-1;0<=(cnt);--(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define diterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);--(cnt))
const ll MD = 1000000007ll; const long double PI = 3.1415926535897932384626433832795L;
inline void assert_call(bool assertion, function<void()> f) { if (!assertion) { cerr << "assertion fault:" << endl; f(); abort(); } }
template<typename T1, typename T2> inline ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << '(' << p.first << ':' << p.second << ')'; return o; }
template<typename Vec> inline ostream& _ostream_vecprint(ostream& os, const Vec& a) {
    os << '['; for (const auto& e : a) os << ' ' << e << ' '; os << ']'; return os;
}
template<typename T> inline ostream& operator<<(ostream& o, const vector<T>& v) { return _ostream_vecprint(o, v); }
template<typename T, size_t S> inline ostream& operator<<(ostream& o, const array<T, S>& v) { return _ostream_vecprint(o, v); }
template<typename T> inline T& maxset(T& to, const T& val) { return to = max(to, val); }
template<typename T> inline T& minset(T& to, const T& val) { return to = min(to, val); }
void bye(string s, int code = 0) { cout << s << endl; exit(code); }
mt19937_64 randdev(8901016);
template<typename T> inline T rand(T l, T h) { return uniform_int_distribution<T>(l, h)(randdev); }
template<> inline double rand<double>(double l, double h) { return uniform_real_distribution<double>(l, h)(randdev); }
template<> inline float rand<float>(float l, float h) { return uniform_real_distribution<float>(l, h)(randdev); }

#if defined(_WIN32) || defined(_WIN64)
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#elif defined(__GNUC__)
#else
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
namespace {
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
    class MaiScanner {
    public:
        template<typename T> void input_integer(T& var) {
            var = 0; T sign = 1;
            int cc = getchar_unlocked();
            for (; cc < '0' || '9' < cc; cc = getchar_unlocked())
                if (cc == '-') sign = -1;
            for (; '0' <= cc && cc <= '9'; cc = getchar_unlocked())
                var = (var << 3) + (var << 1) + cc - '0';
            var = var * sign;
        }
        inline int c() { return getchar_unlocked(); }
        inline MaiScanner& operator>>(int& var) { input_integer<int>(var); return *this; }
        inline MaiScanner& operator>>(long long& var) { input_integer<long long>(var); return *this; }
        inline MaiScanner& operator>>(string& var) {
            int cc = getchar_unlocked();
            for (; !isvisiblechar(cc); cc = getchar_unlocked());
            for (; isvisiblechar(cc); cc = getchar_unlocked())
                var.push_back(cc);
            return *this;
        }
        template<typename IT> void in(IT begin, IT end) { for (auto it = begin; it != end; ++it) *this >> *it; }
    };
    class MaiPrinter {
    public:
        template<typename T>
        void output_integer(T var) {
            if (var == 0) { putchar_unlocked('0'); return; }
            if (var < 0)
                putchar_unlocked('-'),
                var = -var;
            char stack[32]; int stack_p = 0;
            while (var)
                stack[stack_p++] = '0' + (var % 10),
                var /= 10;
            while (stack_p)
                putchar_unlocked(stack[--stack_p]);
        }
        inline MaiPrinter& operator<<(char c) { putchar_unlocked(c); return *this; }
        inline MaiPrinter& operator<<(int var) { output_integer<int>(var); return *this; }
        inline MaiPrinter& operator<<(long long var) { output_integer<long long>(var); return *this; }
        inline MaiPrinter& operator<<(char* str_p) { while (*str_p) putchar_unlocked(*(str_p++)); return *this; }
        inline MaiPrinter& operator<<(const string& str) {
            const char* p = str.c_str();
            const char* l = p + str.size();
            while (p < l) putchar_unlocked(*p++);
            return *this;
        }
        template<typename IT> void join(IT begin, IT end, char sep = ' ') { for (bool b = 0; begin != end; ++begin, b = 1) b ? *this << sep << *begin : *this << *begin; }
    };
}
MaiScanner scanner;
MaiPrinter printer;


namespace BitAlgo {
    // 1が立っている数(f(1) = 1, f(6) = 2)
    inline int popcount32(uint32_t v) noexcept {
#if defined(__GNUC__)
        return __builtin_popcount(v);
#elif defined(_MSC_VER)
        return __popcnt(v);
#else
        v = (v & 0x55555555) + (v >> 1 & 0x55555555);
        v = (v & 0x33333333) + (v >> 2 & 0x33333333);
        v = (v & 0x0f0f0f0f) + (v >> 4 & 0x0f0f0f0f);
        v = (v & 0x00ff00ff) + (v >> 8 & 0x00ff00ff);
        return (v & 0x0000ffff) + (v >> 16 & 0x0000ffff);
#endif
    }
    // 1が立っている数(f(1) = 1, f(6) = 2)
    inline int popcount64(uint64_t v) noexcept {
#if defined(__GNUC__)
        return __builtin_popcountll(v);
#elif defined(_MSC_VER)
        return __popcnt64(v);
#else
        v = (v & 0x5555555555555555ll) + (v >> 1 & 0x5555555555555555ll);
        v = (v & 0x3333333333333333ll) + (v >> 2 & 0x3333333333333333ll);
        v = (v & 0x0f0f0f0f0f0f0f0fll) + (v >> 4 & 0x0f0f0f0f0f0f0f0fll);
        v = (v & 0x00ff00ff00ff00ffll) + (v >> 8 & 0x00ff00ff00ff00ffll);
        v = (v & 0x0000ffff0000ffffll) + (v >> 16 & 0x0000ffff0000ffffll);
        return (v & 0x00000000ffffffffll) + (v >> 32 & 0x00000000ffffffffll);
#endif
    }
    // popcount % 2
    inline int parity32(uint32_t v) noexcept {
#if defined(__GNUC__)
        return __builtin_parity(v);
#else
        return popcount32(v) & 1;
#endif
    }
    // popcount % 2
    inline int parity64(uint64_t v) noexcept {
#if defined(__GNUC__)
        return __builtin_parityll(v);
#else
        return popcount32(v) & 1;
#endif
    }
    // 2進で表した場合に小さい方から何桁目に初めて1が現れるか
    // (f(3) = 1, f(8) = 4, f(0) = 0)
    // 関連：__builtin_ctz
    inline int ffs32(uint32_t v) noexcept {
#if defined(__GNUC__)
        return __builtin_ffs(v);
#else
        int c = 1;
        if (v == 0) return 0;
        if (!(v & 0xFFFF)) c += 16, v >>= 16;
        if (!(v & 0x00FF)) c += 8, v >>= 8;
        if (!(v & 0x000F)) c += 4, v >>= 4;
        if (!(v & 0x0003)) c += 2, v >>= 2;
        if (!(v & 0x0001)) c += 1;
        return c;
#endif
    }
    // 2進で表した場合に小さい方から何桁目に初めて1が現れるか
    // (f(3) = 1, f(8) = 4, f(0) = 0)
    // 関連：__builtin_ctzll
    inline int ffs64(uint64_t v) noexcept {
#if defined(__GNUC__)
        return __builtin_ffsll(v);
#else
        int c = 1;
        if (v == 0) return 0;
        if (!(v & 0xFFFFFFFFll)) c += 32, v >>= 32;
        if (!(v & 0x0000FFFFll)) c += 16, v >>= 16;
        if (!(v & 0x00FF)) c += 8, v >>= 8;
        if (!(v & 0x000F)) c += 4, v >>= 4;
        if (!(v & 0x0003)) c += 2, v >>= 2;
        if (!(v & 0x0001)) c += 1;
        return c;
#endif
    }
    // 末尾から続く0の個数
    // (f(2) = 30, f(0) = 32)
    inline int clz32(uint32_t x) noexcept {
#if defined(__GNUC__)
        return x == 0 ? 32 : __builtin_clz(x);
        // #elif defined(_MSC_VER)
        //         return __lzcnt(x); // _BitScanReverse
#else
        union {
            uint32_t i;
            float f;
        } data;
        data.f = (float)x + 0.5;
        return 158 - (data.i >> 23);
#endif
    }
    // 末尾から続く0の個数
    // (f(2) = 62, f(0) = 64)
    inline int clz64(uint64_t x) noexcept {
#if defined(__GNUC__)
        return x == 0 ? 64 : __builtin_clzll(x);
        // #elif defined(_MSC_VER)
        //         return __lzcnt64(x);
#else
        union {
            uint64_t i;
            double f;
        } datad;
        datad.f = (double)x + 0.5;
        return int(1086 - (datad.i >> 52));
#endif
    }
}


int N;
ll A[200010];


int main() {

    scanner >> N;
    scanner.in(A, A + N);

    map<ll, int> stock;

    ll ans = 0;

    repeat(i, N) {
        ll a = A[i];
        // ll mask = ((1ll << (64 - BitAlgo::clz64(a))) - 1);
        // ll inva = a ^ mask;
        // debug(a);
        // debug(inva);
        // debugv(stock);
        stock[a - 1]++;
    }

    // sort(A, A + N, [](ll a, ll b){
    //     ll mask;
    //     mask = ((1ll << (64 - BitAlgo::clz64(a))) - 1);
    //     ll inva = a ^ mask;
    //     mask = ((1ll << (64 - BitAlgo::clz64(b))) - 1);
    //     ll invb = b ^ mask;
    //     return inva < invb;
    // });
    
    sort(A, A+N);
    
    rrepeat(i, N) {
        ll a = A[i];
        ll mask = (1ll << (64 - BitAlgo::clz64(a))) - 1;
        ll inva = a ^ mask;
        // assert(a >= inva);
        // debug(a);
        // debug(inva);
        // debugv(stock);
        
        if (stock.count(inva) && stock[inva] >= 1 + (inva == a-1) && stock[a-1] >= 1) {
            stock[inva]--;
            stock[a-1]--;
            ans += 1;
        }
    }

    printer << ans << '\n';

    return 0;
}
/*
8
1 1 1 1 7 9 7 9
*/

Measured memory_bytes: 14336