import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        char[] arr = sc.next().toCharArray();
        int count = 0;
        ArrayList<Integer> list = new ArrayList<>();
        for (char c : arr) {
            if (c == '3' || c == '5' || c == '7') {
                count++;
            } else {
                list.add(c - '0');
            }
        }
        int oneCount = 0;
        int nineCount = 0;
        for (int x : list) {
            if (x == 1) {
                oneCount++;
            } else {
                if (oneCount > 0) {
                    count++;
                    oneCount--;
                } else {
                    nineCount++;
                }
            }
        }
        count += oneCount / 2;
        System.out.println(count);
    }
}