Number Pyramid

Image a number pyramid starts with 1, and the numbers increasing by 1.

For example, when the pyramid has 3 levels:

  1
 2 3
4 5 6

And the sum of three corners are:

1 + 4 + 6 = 11

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Input:

You will be given a number n, which means how many levels the pyramid has.

0 <= n <= 5000

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Output:

You need to return the sum of three corners of the pyramid.

• When there is no level, return 0 as there is no corner.

• When there is only one level, return 1 as it is the only corner.

Examples:

sum_corners(0) --> 0
sum_corners(1) --> 1
sum_corners(2) --> 6
sum_corners(3) --> 11

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Golfing Message:

The length of your code should be less or equal to 35.

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If you like this series, welcome to do other kata of it.