Suppose $R$ is a Noetherian ring of Krull dimension $1$ with the property $\dim I+\operatorname{codim} I=\dim R$ for every prime ideal $I$. Suppose $a\in R$ is a nonzerodivisor. If $P$ is a 1-codimensional prime of $(a)\subset P$ we have
$$\mathrm{length}(R_P/(a))=\sum_{Q\subset P\text{ a minimal prime}}\mathrm{length}(R_Q)\mathrm{length}(R/((a)+Q)).$$
Here we define length of an $R$-module $M$ to be the least length of a composition series. A chain of submodules of $M$ is a sequence of submodules with strict inclusions $M=M_0\supset M_1\supset \dots \supset M_n$ is said to be a composition series if each $M_i/M_{i+1}$ is a nonzero simple module, i.e. a composition series is the maximal chain of submodules of $M$.
I am not sure how to deal with it, the thing is, those terms are modules over different rings, any suggestions?