What is the equilibrium expression for the reaction ${\text{P}}_4(s)+5{\text{O}}_2(g)\rightleftharpoons {\text{P}}_4{\text{O}}_{10}(s)$?

In writing equilibrium constant expressions, the activities (concentrations) of pure solids and pure liquids are taken as unity ($1$).
Reaction: ${\text{P}}_4(s)+5{\text{O}}_2(g)\rightleftharpoons {\text{P}}_4{\text{O}}_{10}(s)$.
$K_c=\frac{[{\text{P}}_4{\text{O}}_{10}(s)]}{[{\text{P}}_4(s)][{\text{O}}_2(g){]}^5}=\frac{1}{1\times [{\text{O}}_2{]}^5}=1/[{\text{O}}_2{]}^5$.