def PC_AC1_calc(P, TOP, POP):
    """
    Calculate percent chance agreement for Gwet's AC1.

    :param P: condition positive
    :type P : dict
    :param TOP: test outcome positive
    :type TOP : dict
    :param POP: population
    :type POP:dict
    :return: percent chance agreement as float
    """
    try:
        result = 0
        classes = list(P.keys())
        for i in classes:
            pi = (P[i] + TOP[i]) / (2 * POP[i])
            result += pi * (1 - pi)
        result = result / (len(classes) - 1)
        return result
    except Exception:
        return 'None'
