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];
		ans += cnt[1] / 2;
		if (cnt[1] % 2 == 1 && cnt[9] > 0) {
			ans++;
		}
		System.out.println(ans);
	}
}