import java.io.*;
import java.util.*;
 
public class P51 {
 
    public static void main(String[] args) throws IOException {
        Scanner io = new Scanner(System.in);
        int n = io.nextInt();
        long ans = (long) n * (n + 1) / 2;
        while (n-- > 1) {
            int a = io.nextInt();
            ans -= a;
        }
        System.out.println(ans);
    }
}