import java.util.Scanner;
public class Main{
	private static Scanner sc=new Scanner(System.in);
	public static void main(String args[])throws Exception{
		boolean are[]=new boolean[10];
		for(int i=0;i<are.length-1;i++){
			are[sc.nextInt()-1]=true;
		}for(int i=0;i<are.length;i++){
			if(are[i]==false){
				System.out.println(i+1);break;
			}
		}
	}
}