def concatSC(m, n, k=1):
    """Worst case state complecity for concatenation
    :arg m: number of states
    :arg n: number of states
    :arg k: number of letters
    :type m: integer
    :type n: integer
    :type k: integer
    :returns: state compelxity
    :rtype: integer"""

    return m * 2 ** n - k * 2 ** (n - 1)