I have the following nested sum that is mentioned in a statistic textbook without any justification. The transformation really doesnt seem obvious to me. Could someone maybe elaborate how this holds?

$$ C sum_{k=2}^{infty} sum_{j=1}^{k-1}left(frac{1}{2}right)^{k}=C sum_{j=1}^{infty} sum_{k=j+1}^{infty}left(frac{1}{2}right)^{k}$$