at room temperature dinitrogen tetroxide is in equilibrium with nitrogen dioxide. Deduce an expression relating Kp for decomposition reaction with "a" the degree of dissociation and "P" the total pressure.

I have tried it out using 100 as an initial number of moles of dinitrogen tetroxide but I am still in doubt. I got this answer:

(200aP)/(1-100a) = Kp But is this relevant for when you start only with 100?

Have I worked it right?

Thanks in advance for your help