Connecting start and end points of an interpolated curve

I am just learning the fundamentals of Grasshopper, and have been working my way through the Generative Algorithms with Grasshopper version 2.0, eBook available in the tutorials section. 

As visible in the attached screenshots, I have recreated one of the basic definitions discussed in the book, but cannot manage to get the first and final point of my interpolated curve to connect as with the rest of the curve.

I'd appreciate any friendly direction. 

Thanks