import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        primeSubSequence solver = new primeSubSequence();
        solver.solve(1, in, out);
        out.close();
    }

    static class primeSubSequence {
        public void solve(int testNumber, Scanner in, PrintWriter out) {

        /*
        3,5,7はすべて取り除いていい
        考えるべきなのは
        - 11
        - 19
        をどうつくるか。
        まず19を作ってから残りは11
        左からの1の個数と右からの9の個数を考えていってminをとってそのmaxをとればいい。
        残った1の数//2でわかるようになる
         */

            int n = in.nextInt();
            String s = in.next();
            int l = s.length();
            int res = 0;
            int[] one = new int[l + 1];
            int[] nin = new int[l + 1];
            int max = -1;
            for (int i = 0; i < l; i++) {
                if (s.charAt(i) == '3' || s.charAt(i) == '5' || s.charAt(i) == '7') {
                    res++;
                }
            }

            one[0] = 0;
            for (int i = 0; i < l; i++) {
                if (s.charAt(i) == '1') {
                    one[i + 1] = one[i] + 1;
                } else {
                    one[i + 1] = one[i];
                }
            }

            nin[0] = 0;
            for (int i = l - 1; i >= 0; i--) {
                if (s.charAt(i) == '9') {
                    nin[i + 1] = nin[i] + 1;
                } else {
                    nin[i + 1] = nin[i];
                }
            }

            for (int i = 0; i < n; i++) {
                max = Math.max(max, Math.min(one[i], nin[n - i]));
            }

            out.println(res + max + (one[n] - max) / 2);


        }

    }
}