import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int n=0;
        int result=0;

        for(int i=1; i<=9; i++){
            n=sc.nextInt();
        if(i != n){
            result=n-1;
            break;
        }
    }
        System.out.println(result);
            sc.close();
		}
    }