Imagine we create a pandas series using the below code. What is one simple way to retrieve the value $0\mathrm{.}5$ from the series?

import pandas as pd

df $=$ pd$.$Series $,$

index $($:$={[}^{\text{'}}{a}^{\text{'}},{?}^{\text{'}}{b}^{\text{'}},{?}^{\text{'}}{c}^{\text{'}},{?}^{\text{'}}{d}^{\text{'}}]\}$

df