I have to verify these games are convex-concave:

a) $\displaystyle f(p,q)=-(pq)^2$ f(p,q)=-(pq)^2

b) $\displaystyle f(p,q)=-(p^2-2p+1)(q^2+4q+5)$ f(p,q)= -(p^2-2p+1)(q^2+4q+5)

I am a bit confused as i thought i was supposed to take the second derivative with respect to p, show it is positive then take the second derivative with respect to q and show it is negative but i get that both of these are negative.

Thanks for any help