MainPage.xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:d="http://xamarin.com/schemas/2014/forms/design" 
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d" 
             x:Class="XamarinFormsCommandDemo.MainPage"
             xmlns:vm="clr-namespace:XamarinFormsCommandDemo">
    <ContentPage.BindingContext>
        <vm:MainViewModel/>
    </ContentPage.BindingContext>
    <StackLayout VerticalOptions="Center" HorizontalOptions="CenterAndExpand">
       <Entry Text="{Binding Number}"/>
        <Button Text="Calculate" Command="{Binding CalculateCommand}"/>
        <Label Text="{Binding Result}" FontSize="Large"/>
    </StackLayout>
</ContentPage>