How to sum all price values having in count the quantity of each one

Using pandas, I would like to sum all of the price to get the count value for each one. The Dataframe looks like this:

   price  quantity
0     25         2
1     20         3
2     20        -1


My code:

import pandas as pd

df = pd.DataFrame({'price': [25, 20, 20], 'quantity': [2, 3, -1]})
print(df)


So, I would expect the output to be something like $$(25*2)+(20*3)+(20*-1)$$:

>>> 90


values = df['price'] * df['quantity']