import java.util.Scanner;
public class Test15 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String ss = sc.nextLine();

		String[]s =ss.split(" ");
		int c=1;
		for(int i=1;i<=10;i++){
			int a = Integer.parseInt(s[i-1]);
			if(a!=c){
				System.out.println(a-1);
				break;
			}else if(a==9){
				System.out.println(a+1);
				break;
			}
			c++;
		}
	}
}