import java.util.*;
import java.util.Arrays;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int a1 = 21;//sc.nextDouble();
		int a2 = 9;//sc.nextInt();
		int a3 = 3;//sc.nextInt();
		System.out.println((a3*(a3-a1)+a2*(a2-a3))/(a2-a1));
	}
}