找出缺失的数

等差数列求和，减去已输入的数字的和，差即为未出现的：

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);
    }
}