quantum field theory - Geometric series for two-point function

In deriving the expression for the exact propagator

$$G_c^{(2)}(x_1,x_2)=[p^2-m^2+\Pi(p)]^{-1}$$

for $\phi^4$ theory all books that i know use the following argument:

$$G_c^{(2)}(x_1,x_2)=G_0^{(2)}+G_0^{(2)}\Pi G_0^{(2)}+G_0^{(2)}\Pi G_0^{(2)}\Pi G_0^{(2)}+...$$

wich is a geometric series so the formula for the exact propagator.Here

$$G_0^{(2)}$$

is the free propagator and

$$\Pi=X+Y+Z+...+W$$

is the sum of all irreducible diagrams.Here the irreducible diagrams is represented by $X,Y,Z,...,W$.

Using the path integral i can see, that connected diagrams $D$ can be written in the form

$$D=G_0^{(2)}XG_0^{(2)}Z...G_0^{(2)}W$$

Question: But how to prove that there is no constant $C$ so that instead we have

$$D=G_0^{(2)}XG_0^{(2)}Z...G_0^{(2)}W$$

we would have

$$D=CG_0^{(2)}XG_0^{(2)}Z...G_0^{(2)}W~?$$

In the last case we would not have a geometrical serie. Can someone explain me i t please or give me another way to derive it.