The K-th Hexagonal Number

Difficulty: 1100 2000 ms 128 MB

math

Print the K-th of the hexagonal numbers (1, 6, 15, 28, ...) (1-indexed).

Input: one integer K.

Output: the K-th such number.

Constraints:

• 1 ≤ K ≤ 1000000000

Sample Tests

3

15

1

1

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