题解 | 牛牛学数列

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 scan = new Scanner(System.in);
        int n = scan.nextInt();
        int sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;//加一倍i
            if (i % 2 == 0){
               sum = sum - 2 * i;//见两倍i
            }
        }
        System.out.println(sum);

    }
}