How to use CalcBinding in XAML

Question

I have two text boxes Quantity and PricePerItem and a TextBlock named as TotalTb. What i want is multiply Quantity with PricePerItem and reflect result in TextBlock using CalcBinding.
Will i've to create respective properties at backend ? or Apply INotifyPropertyChanged for simple calculations like this ?
Not using MVVM. What's the simplest way to achieve this ?

<TextBox x:Name="Quantity"/>
<TextBox x:Name="PricePerItem"/>
<TextBlock x:Name="TotalTb"/>