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();
        ArrayList<Integer> list = new ArrayList<>();
        int count = 0;
        for (char c : arr) {
            if (c == '3' || c == '5' || c == '7') {
                count++;
            } else {
                list.add(c - '0');
            }
        }
        int length = list.size();
        int[] sums1 = new int[length];
        for (int i = 0; i < length; i++) {
            if (i != 0) {
                sums1[i] = sums1[i - 1];
            }
            if (list.get(i) == 1) {
                sums1[i]++;
            }
        }
        int max = 0;
        int sum9 = 0;
        for (int i = length - 1; i >= 0 && sum9 <= sums1[i]; i--) {
            max = sum9;
            if (list.get(i) == 9) {
                sum9++;
            }
        }
        count += max;
        if (length > 0) {
            count += (sums1[length - 1] - max) / 2;
        }
        System.out.println(count);
  }
}