Your mission is to convert the name of a function (a string) from CamelCase ("MyFunctionName") into the Python format ("my_function_name") where all chars are in lowercase and all words are concatenated with an intervening underscore symbol "_".

Input: A function name as a CamelCase string.

Output: The same string, but in under_score.

fromCamelCase('MyFunctionName') == 'my_function_name'
fromCamelCase('IPhone') == 'i_phone'

How it is used: To apply function names in the style in which they are adopted in a specific language (Python, JavaScript, etc.).

Precondition:
0 < len(string) <= 100
Input data won't contain any numbers.