def compute_r_parameter(ago_susceptible: int, current_susceptible: int, current_infected: int) -> float:
    """
    Function for compute infection ratio parameter based od SIR model.

    Parameters
    ----------
    ago_susceptible: int, required
        Amount of susceptible people one interval before current iteration.
    current_susceptible: int, required
        Amount of susceptible people in current interval.
    current_infected: int, required
        Amount of infected people in current interval.

    Returns
    -------
    Returns value of infection ratio.

    Example
    -------
    >>> compute_r_parameter(10, 9, 1)
    """

    d_susceptible = current_susceptible - ago_susceptible

    return -d_susceptible * (1 / (current_susceptible * current_infected))