how to implement observable int in wpf ViewModel?

While we're at it with improving the answer, some of the other new additions of c# 6.0 and 7.0 help make it ever more compact:

public class Prop<T> : INotifyPropertyChanged
{
    private T _value;

    public T Value
    {
        get => _value; 
        set { _value = value; NotifyPropertyChanged(nameof(Value)); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    internal void NotifyPropertyChanged(string propertyName) => 
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

This way, you aren't using any "embedded values" (i.e - the property's name) and are keeping the code refactor-safe.

And there's also no need for redundant code blocks due to c# 6.0 and 7.0's new Expression body features

You have two choices:

1. Implement the INotifyPropertyChanged interface on your class.
2. Inherit from DependencyObject and implement Delta as a DependencyProperty.

The simplest option is #1. You can implement the INotifyPropertyChanged interface on your class quite easily:

public class YourClass : INotifyPropertyChanged
{

  private int _delta;
  public int Delta
  {
      get { return _delta; }
      set { _delta = value; PropertyChanged?.Invoke(nameof(Delta)); }
  }

  public event PropertyChangedEventHandler PropertyChanged;
}

You can read more about using and implementing dependency properties on MSDN.

Using @LBushKin's Answer, i modified it to

public class Prop<T> : INotifyPropertyChanged
{
    private T _value;
    public T Value
    {
        get { return _value; }
        set { _value = value; NotifyPropertyChanged("Value"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    internal void NotifyPropertyChanged(String propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

and to set it up:

class MainWindow ...
    // a bool with initial value of true
    public static Prop<bool> optionBool { get; set; } = new Prop<bool>{ Value = true };

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        // connect UI to be able to use the Prop
        DataContext = this;
    }

and to use it:

<Grid ...
    <CheckBox Content="Da Check" ... IsChecked="{Binding optionBool.Value}"/>

There is also a Collection and 2-Properties version here: Utils.ObservableProperties.cs (this repo contains several related classes)