import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String s = br.readLine();
		br.close();

		int[] cnt = new int[10];
		for (int i = 0; i < n; i++) {
			cnt[s.charAt(i) - '0']++;
		}
		int ans = 0;
		ans += cnt[3];
		ans += cnt[5];
		ans += cnt[7];

		int cnt1 = 0;
		for (int i = 0; i < n; i++) {
			if (s.charAt(i) == '1') {
				cnt1++;
			} else if (s.charAt(i) == '9') {
				if (cnt1 > 0) {
					ans++;
					cnt1--;
				}
			}
		}
		ans += cnt1 / 2;
		System.out.println(ans);
	}
}