import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        
        int lostNumber=0;
        for(int i=1;i<=9;i++){
            int number=sc.nextInt();
            if(number!=i){
                lostNumber=i;
                break;
            }
        }
        if(lostNumber==0){
            lostNumber=10;
        }
        System.out.println(lostNumber);
    }
}