[Ifeffit] Linear combination of Sigma ^ 2

Dear all, I am following Bruce’s video presentation on the Artemis case of FeS2, and have a beginner’s question here. There is a configuration of S1 ---- Fe ---- S2 . In order to use the existing sigma^2 defined for single scattering path of [Fe – S], Bruce applied linear combination for the following paths: Fe ---- S1 ---- S2 ---- Fe Sigma^2 of this path = sigma^2 [Fe – S] *2 Fe ---- S1 ---- Fe ---- S2 ---- Fe Sigma^2 of this path = sigma^2[Fe – S) *2 Fe ---- S1---- Fe ---- S1 ---- Fe Sigma^2 of this path = sigma^2 [Fe – S] *4 Hence the question is, why the third path multiplies 4 while the other two paths multiply 2? For me they should all be 2. Thanks. Regards, Zhipeng