MainWindow.xaml

<Window x:Class="WpfReactiveUIExample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="164" Width="180">
    <Grid Margin="6">
        <StackPanel>
            <TextBox Text="{Binding AmountA, UpdateSourceTrigger=PropertyChanged}" Height="23" TextWrapping="Wrap" Width="120"/>
            <TextBox Text="{Binding AmountB, UpdateSourceTrigger=PropertyChanged}" Height="23" TextWrapping="Wrap" Width="120"/>
            <TextBox Text="{Binding AmountC, UpdateSourceTrigger=PropertyChanged}" Height="23" TextWrapping="Wrap" Width="120"/>
            <TextBlock Text="{Binding Total, UpdateSourceTrigger=PropertyChanged}" TextWrapping="Wrap" Height="20" Width="120"/>
            <Button Content="Button" Command="{Binding OkCommand}" Width="75"/>
        </StackPanel>
    </Grid>
</Window>