import math


def pwait(c, rho):
    """
    Probability that a job has to wait

    :param c: amount of servers
    :param rho: occupation rate per server
    :return: probability
    """
    fixed = ((c*rho)**c)/math.factorial(c)

    sum = 0
    for n in range(c):
        sum += ((c*rho)**n)/math.factorial(n)

    return fixed/((1-rho)*sum + fixed)