#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"
template<class T> void chmax(T &a, const T b){ a = max(a, b); }
template<class T> void chmin(T &a, const T b){ a = min(a, b); }

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    const ll inf = LONG_LONG_MIN / 2 - 1;
    ll n;
    cin >> n;
    string s;
    cin >> s;
    // 3, 5, 7は単品で使うのが良い
    string ns = "";
    rep(i, n) {
        if ((s[i] == '3') || (s[i] == '5') || (s[i] == '7')) continue;
        ns += s[i];
    }
    ll ans = n - len(ns);
    // 1と9を組み合わせて使う: 11, 19, 991
    n = len(ns);
    s = ns;
    ll one = 0, nine = 0;
    rep(i, n) {
        if (s[i] == '1') {
            if (nine >= 2) {
                nine -= 2;
                ans++;
            }
            else if (one >= 1) {
                one--;
                ans++;
            }
            else {
                one++;
            }
        }
        else {
            if (one >= 1) {
                one--;
                ans++;
            }
            else {
                nine++;
            }
        }
    }
    cout << ans << endl;
    return 0;
}