题解 | #等差数列#
等差数列
https://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f?tpId=37&tqId=21323&rp=1&ru=/exam/oj/ta&qru=/exam/oj/ta&sourceUrl=%2Fexam%2Foj%2Fta%3FtpId%3D37&difficulty=undefined&judgeStatus=undefined&tags=&title=100
n = int(input()) # Sn = a1 * n + (n - 1) * d * n / 2 s = 2 * n + (n - 1) * n * 3 // 2 print(s)
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