//#pragma GCC optimize("Ofast")
//#pragma GCC target("avx")
//#undef LOCAL
#include <bits/stdc++.h>

using namespace std;
using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n - 1); }
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;

#ifdef LOCAL
struct PrettyOS {
    ostream& os;
    bool first;
    template <class T> auto operator<<(T&& x) {
        if (!first) os << ", ";
        first = false;
        os << x;
        return *this;
    }
};
template <class... T> void dbg0(T&&... t) {
    (PrettyOS{cerr, true} << ... << t);
}
#define dbg(...)                                            \
    do {                                                    \
        cerr << __LINE__ << " : " << #__VA_ARGS__ << " = "; \
        dbg0(__VA_ARGS__);                                  \
        cerr << endl;                                       \
    } while (false);
#else
#define dbg(...)
#endif

template <class T, class U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    return os << "P(" << p.first << ", " << p.second << ")";
}

template <class T> ostream& operator<<(ostream& os, const V<T>& v) {
    os << "[";
    for (auto d : v) os << d << ", ";
    return os << "]";
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
    int n;
    string s;
    cin >> n >> s;

    int ans = 0;
    V<int> used(n);

    for (int i = 0; i < n; i++) {
        if (s[i] == '3' || s[i] == '7' || s[i] == '5') {
            used[i] = true;
            ans++;
            continue;
        }
    }

    {
        //19
        V<int> ones;
        for (int i = 0; i < n; i++) {
            if (used[i]) continue;
            if (s[i] == '1') {
                ones.push_back(i);
            } else {
                if (ones.size()) {
                    int j = ones.back();
                    ones.pop_back();
                    used[j] = true;
                    used[i] = true;
                    ans++;
                }
            }
        }
    }

    {
        //991
        V<int> nines;
        for (int i = 0; i < n; i++) {
            if (used[i]) continue;
            if (s[i] == '1') {
                if (nines.size() >= 2) {
                    int j0 = nines.back(); nines.pop_back();
                    int j1 = nines.back(); nines.pop_back();
                    used[j0] = used[j1] = used[i] = true;
                    ans++;
                }
            } else {
                nines.push_back(i);
            }
        }
    }
    int oc = 0;
    for (int i = 0; i < n; i++) {
        if (used[i]) continue;
        if (s[i] == '1') oc++;
    }

    ans += oc / 2;
    cout << ans << endl;
    return 0;
}