Oh, Rxb4
ok
Kc2
Za5
Rxc5
Rc4+
Rxc4
Zb4+
Kb3
That is illegal because you get turned into a pawn.
Kd3
That is illegal because I can turn your king into a pawn.
Kc1
Zc3#
GGFinal Position
gg
Oh, Rxb4