import java.util.Scanner;
class Wandbox
{
    public static void main(String[] args)
    {
    	Scanner sc = new Scanner(System.in);
    	
    	for (int i = 1; i <= 9; i++) {
    		if (sc.nextInt() != i) {
    			System.out.println(i);
    			break;
    		} else if (i == 9) {
    			
    			System.out.println(10);
    		}
    	} 
    	
    }
}