import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int idx = 8;
        while (idx + 7 <= n) {
            if (idx + 7 == n) {
                System.out.println(idx);
                return;
            }
            idx++;
        }
        System.out.println(-1);
  }
}