题解 | #等差数列#

while True:
    try:
        num = int(input())
        sum_num = 2
        b = [2] #初始为2
        for i in range(1,num): #从等差数列第二个数，即5开始放入b
            sum_num += 3
            b.append(sum_num)
        sum_num = 0 #sum_num 归零
        for i in range(len(b)):
            sum_num += b[i] #等差数列求和
        print(sum_num)
    except:
        break