题解 | 小乐乐求和

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        long a = in.nextLong();
        long sum = 0;
        sum = a * (a + 1) / 2;
        System.out.println(sum);
    }
}