There are `3n` piles of coins of varying size, you and your friends will take piles of coins as follows:

- In each step, you will choose any `3` piles of coins (not necessarily consecutive).
- Of your choice, Alice will pick the pile with the maximum number of coins.
- You will pick the next pile with the maximum number of coins.
- Your friend Bob will pick the last pile.
- Repeat until there are no more piles of coins.

Given an array of integers `piles` where `piles[i]` is the number of coins in the `ith` pile.

Return the maximum number of coins that you can have.

Example 1:

    Input: piles = [2,4,1,2,7,8]
    Output: 9
    Explanation: Choose the triplet (2, 7, 8), Alice Pick the pile with 8 coins, you the pile with 7 coins and Bob the last one.
    Choose the triplet (1, 2, 4), Alice Pick the pile with 4 coins, you the pile with 2 coins and Bob the last one.
    The maximum number of coins which you can have are: 7 + 2 = 9.
    On the other hand if we choose this arrangement (1, 2, 8), (2, 4, 7) you only get 2 + 4 = 6 coins which is not optimal.

Example 2:

    Input: piles = [2,4,5]
    Output: 4

Example 3:

    Input: piles = [9,8,7,6,5,1,2,3,4]
    Output: 18

Constraints:

- `3 <= piles.length <= 105`
- `piles.length % 3 == 0`
- `1 <= piles[i] <= 104`