因为没有重复的数，所以等差数列求和，减去输入的总和，剩下的就是为未出现的数

import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int sum = 0;
        while(sc.hasNext()) sum += sc.nextInt();
        System.out.println((1 + n)*n/2 - sum);
    }
}