题解 | #序列找数#

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int sum = ((n + 1) * n) >> 1;
        for(int i = 0; i < n; i++) {
            sum -= sc.nextInt();
        }
        System.out.println(sum);
    }
}