NOTE: It is recommended to complete the corner case before working on this edge case.

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Number Pyramid:

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

For example, when the pyramid has 4 levels:

   1
  2 3
 4 5 6
7 8 9 10

And the sum of all numbers of the edges (including corners) are:

1 + 2 + 3 + 4 + 6 + 7 + 8 + 9 + 10 = 50 (5 is not on the edges)

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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 all numbers on the edges (corners are included) of the pyramid.

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

• When there is only one level, return 1 as it is the only number on edge/corner.

Examples:

sum_edges(0) --> 0 or 0.0
sum_edges(1) --> 1 or 1.0
sum_edges(2) --> 6 or 6.0
sum_edges(3) --> 21 or 21.0
sum_edges(4) --> 50 or 50.0

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Golfing Message:

The length of your code should be less or equal to 50.

It is possible to do it within the length of 46.

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