import java.util.*;
import java.lang.*;
import java.io.*;

// The main method must be in a class named "Main".
class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int[] arr=new int[n];
        for(int i=0;i<n;i++){
            arr[i]=sc.nextInt();
        }
        int sum=0;
        int sum_arra=((n)*(n+1))/2;
        for(int i=0;i<n;i++){
            sum+=arr[i];
        }

        System.out.println(sum_arra-sum);
    }
}