Stacking Pentagons

Difficulty: 1100 2000 ms 128 MB

math

Print the sum of the first K pentagonal numbers (1 + 5 + 12 + ...).

Input: one integer K.

Output: K²(K+1)/2.

Constraints:

• 1 ≤ K ≤ 10⁶

Sample Tests

3

18

1

1

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