How do I use nargin to differentiate between a default units setting and unit input from function call

I'm still very new to MATLAB. I need to be able to call my function and if no units are input then it defaults to 'm' like so:

figure(1)
ellipsoid_volume(8,17,'Volume versus Depth for an Ellipsoid')

Otherwise I need to be able to insert the correct units in my function like so:

figure(1)
ellipsoid_volume(8,17,'Volume versus Depth for an Ellipsoid','cm')

How do I use nargin to do this? My homework specifically states, "Use the nargin command to verify the proper number of input arguments and assign units = 'm' if no units are provided."

function ellipsoid_volume(a,b,plot_title,units)
% ellipsoid_volume
% James R. Wallace, JR.
% 30 August 2019
% 
% Calculates volume of horizontal ellipsoid and plots data
% Inputs:
%   a - vertical semi-axis
%   b - horizontal radius
%   plot_title - string holding plot title
%   units - optional string describing units, default = 'm'
units = 'm';    % Default units is meters
h = 0:(3*a);    % Initializing h(height) to index from zero to 3 times a
% Calculating volume of liquid in horizontal ellipsoid
V = (pi/3).*(3*a-h).*((b.^2*h.^2)/a.^2);
% Plotting
yOut = ['Volume [',units,'^3]'];
xOut = ['Depth [',units,']'];
plot(h,V)
ylabel(yOut)
xlabel(xOut)
title(plot_title)

Answer 1

[Stephen23]

Replace

units = 'm';

with

if nargin<4
    units = 'm';
end

[James Wallace]

Thank you.