题解 | #等差数列#
等差数列
https://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
n = int(input()) print(int(n*(2+3*n-1)/2))
等差数列
https://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
n = int(input()) print(int(n*(2+3*n-1)/2))
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