I have the probability-generating function G(s) as following , how got the mentioned Probability-mass function f(k) from it ??

syms u s a k;

G =(1+a*(1+u)-a*(1+u)*s)/((1+u-u*s)*(1+a-a*s));

f(s)=diff(G,s,k);

pmfa=f(0)/factorial(k);

but the result is zero !!