import java.util.Scanner;
public class suuji {
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int result = 0;
		
		for(int i=1; i<10; i++){
			int tmp = sc.nextInt();
			
			if(tmp != i){
				result = i;
				break;
			}
			
		}
		System.out.println(result);
	}

}