等差数列

https://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f?tpId=37&tags=&title=&diffculty=0&judgeStatus=0&rp=1

while True:
    try:
        n = int(input())
        ans = 2*n + (3 * n * (n - 1)) / 2
        print(int(ans))
    except:
        break