#include <bits/stdc++.h>
#pragma GCC diagnostic ignored "-Wsign-compare"
#pragma GCC diagnostic ignored "-Wsign-conversion"

#define NDEBUG
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
using uint = unsigned int;
using usize = std::size_t;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
template<typename T> constexpr T popcount(const T u) { return u ? static_cast<T>(__builtin_popcountll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T log2p1(const T u) { return u ? static_cast<T>(64 - __builtin_clzll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T msbp1(const T u) { return log2p1(u); }
template<typename T> constexpr T lsbp1(const T u) { return __builtin_ffsll(u); }
template<typename T> constexpr T clog(const T u) { return u ? log2p1(u - 1) : static_cast<T>(u); }
template<typename T> constexpr bool ispow2(const T u) { return u and (static_cast<u64>(u) & static_cast<u64>(u - 1)) == 0; }
template<typename T> constexpr T ceil2(const T u) { return static_cast<T>(1) << clog(u); }
template<typename T> constexpr T floor2(const T u) { return u == 0 ? static_cast<T>(0) : static_cast<T>(1) << (log2p1(u) - 1); }
template<typename T> constexpr bool btest(const T mask, const usize ind) { return ((static_cast<u64>(mask) >> ind) & static_cast<u64>(1)); }
template<typename T> constexpr T bcut(const T mask, const usize ind) { return ind == 0 ? static_cast<T>(0) : static_cast<T>((static_cast<u64>(mask) << (64 - ind)) >> (64 - ind)); }
template<typename T> bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); }
template<typename T> bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); }
constexpr unsigned int mod                  = 1000000007;
template<typename T> constexpr T inf_v      = std::numeric_limits<T>::max() / 4;
template<typename Real> constexpr Real pi_v = Real{3.141592653589793238462643383279502884};
template<typename T>
T read()
{
    T v;
    return std::cin >> v, v;
}
template<typename T>
std::vector<T> read_vec(const std::size_t size)
{
    std::vector<T> v(size);
    for (auto& e : v) { std::cin >> e; }
    return v;
}
template<typename... Types>
auto read_vals() { return std::tuple<std::decay_t<Types>...>{read<Types>()...}; }
#define SHOW(...) static_cast<void>(0)
template<typename T>
std::vector<T> make_v(const std::size_t size, T v) { return std::vector<T>(size, v); }
template<class... Args>
auto make_v(const std::size_t size, Args... args) { return std::vector<decltype(make_v(args...))>(size, make_v(args...)); }
int main()
{
    const auto n = read<usize>();
    const auto s = read<std::string>();
    std::string p;
    usize ans = 0;
    for (usize i = 0; i < n; i++) {
        if (s[i] == '3' or s[i] == '5' or s[i] == '7') {
            ans++;
        } else {
            p.push_back(s[i]);
        }
    }
    std::priority_queue<usize> nq;
    std::priority_queue<usize, std::vector<usize>, std::greater<usize>> oq;
    int one = 0, nine = 0;
    for (usize i = 0; i < p.size(); i++) {
        if (p[i] == '1') { oq.push(i), one++; }
        if (p[i] == '9') { nq.push(i), nine++; }
    }
    while (not oq.empty() and not nq.empty()) {
        const usize o = oq.top();
        const usize n = nq.top();
        if (o > n) { break; }
        ans++, one--, nine--;
    }
    const usize use = std::min(nine / 2, one);
    nine -= use * 2, one -= use, ans += use;
    std::cout << ans + one / 2 << std::endl;

    return 0;
}