# Confused with Multivariate time series analysis Book equation

I am reading the Wiley's Multivariate Time Series Analysis Book and there is an step that I don't understand.

At pag. 18 it gets the following equation:

$$\mu = c + \sum_{i=1}^{\infty} \pi_i \mu$$

where $$\mu$$ is a column vector of size $$k$$ and $$c$$ and $$\pi_i$$ are matrices of size $$k$$x$$k$$.

Then it says:

Letting $$\pi_0 = I_k$$, we obtain, from the prior equation,

$$\Big( \sum_{i=0}^{\infty} \pi_i \Big) \mu = c$$.

And I don't know how it gets there...I suppose it is a very simple step but I don't see it.

I'd appreciate if someone can explain it.

Thank you!!