def SIR_model(y, t, betta, gamma):
    """
    Tradiional SIR model.
    :param y:
    :param t:
    :param betta:
    :param gamma:
    :return:
    """
    S,I,R = y
    dS_dt = -betta*S*I
    dI_dt = betta*S*I - gamma*I
    dR_dt = gamma*I
    return ([dS_dt, dI_dt, dR_dt])