Chemical Analysis Chemical Analysis
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As the input you will get the chemical formula and the number N. Your task is to create a list of the chemical elements, which are in the formula in the amount of >= N.
Pay attention, that in the some formulas will be used brackets - () and []. This article will help you to open the brackets and don't make mistake while counting.

Input: Chemical formula and the number of atoms.

Output: List of the chemical elements, sorted in the alphabetical order.

Example:

atoms('C2H5OH', 2) == ['C', 'H']
atoms('H2O', 2) == ['H']
atoms('Mg(OH)2', 1) == ['H', 'Mg', 'O']
atoms('K4[ON(SO3)2]2', 4) == ['K', 'O', 'S']

How it is used: For chemical analysis.

Precondition:
1 <= different elements <= 10