题解 | 牛牛学数列4

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