Moment of inertia of two masses on opposite ends of a rod

[Igottabull]

Rotating a massless rod of length R about the midpoint of the rod with a mass of 2m attached to one end and a mass of m to the other. What is the moment of inertia?

My book is terrible at giving good examples, there's nothing similar to this.

Do I treat them as two separate point particles, take the moment of inertia of each around the axis and add them? (2m*(r/2)^2) + (m*(r/2)^2) I'm assuming I can treat them as points, the drawing has the masses as clearly different sized spheres, but says nothing about their radius or anything of that nature.

Thanks in advance guys.

 

[Igottabull]

I think I have it right, would still appreciate confirmation though, thanks guys.

 

[Harmony]

Your concept is correct, but I think you meant diameter instead of radius for the term r used in your equation.

 

[Igottabull]

Actually the book used R as the length of the rod, I probably shouldn't have used lower case there, sorry about the confusion. Thanks again.