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Background

Description

计算等边三角形使用的火柴根数量（拓展06-计算有多少个三角形[3根火柴组成的三角形]），层次为输入的正整数n（1<=n<=10000）

Format

Input

Output

Samples

1

1

3

9

20

400

Limitation

1s, 1024KiB for each test case.