The equilibrium constant for the following reactions are given at 25°C:
$\mathrm{2A \rightleftharpoons B + C}$, K$_1 = 1.0$
$\mathrm{2B \rightleftharpoons C + D}$, K$_2 = 16$
$\mathrm{2C + D \rightleftharpoons 2P}$, K$_3 = 25$

The equilibrium constant for the reaction $\mathrm{P \rightleftharpoons A + \frac{1}{2}B}$ at 25°C is: