def concatenate_number(a, b):                                       # O(1)
    """
    Concatenates two number together

    >>> concatenate_number(4, 2)
    42
    >>> concatenate_number(-4, 2)
    -42
    >>> concatenate_number(-43, 2)
    -432
    >>> concatenate_number(0, 7)
    7
    """
    if a >= 0:                                                      # O(1)
        return (a * 10) + b                                         # O(1)
    return (a * 10) - b                                             # O(1)