import java.util.*;

class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int sum = 0;
        for(int i=0; i<n; i++) sum += in.nextInt();

        int asm = 0;
        for(int i=1; i<=n; i++) asm += i;

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