#include "bits/stdc++.h"
#define REP(i, n) for(int i = 0; i < int(n); i++)
#define FOR(i,n,m) for(int i = int(n); i < int(m); i++)
using namespace std;
typedef long long ll;
const int MOD = 1e9 + 7;
const int INF = 1e9 + 6;
const ll LLINF = 1e18 + 1;

int main() {
    int n; cin >> n;
    string s; cin >> s;
    int ans = 0;
    bool one = false;
    int cnt_1 = 0, cnt_9_1 = 0, cnt_9_2 = 0;
    REP(i, n) {
        if (s[i] == '3' || s[i] == '5' || s[i] == '7') {
            ans++;
        }
        else if (s[i] == '1') {
            cnt_1++;
            one = true;
        }
        else {
            if (one) {
                cnt_9_2++;
            }
            else {
                cnt_9_1++;
            }
        }
    }
    ans += (cnt_1 >> 1);
    if (cnt_1 & 1 == 1 && (cnt_9_2 >= 1 || cnt_9_1 >= 2)) {
        ans++;
    }
    cout << ans << endl;
    return 0;

}