decorator design pattern for adding new methods and codes to the existing object

I am new in design patterns. I am trying to use decorator design pattern to add new codes and functionalities to my existing app.

Suppose I have a class of App which has two methods "Add" and "Multiply". At some point (run time) the app will require to calculate the average as well. So, I am trying to use decorator design pattern to make this possible.

So far I have :

public class App implements Code{
        public int a=2;
        public int b=3;

    @Override
    public int Add(int a, int b) {
        int add;
        add = a+b;
        return add;
    }

    @Override
    public int Multiply(int a, int b) {
        int mul;
        mul= a*b;
        return mul;
    }

} 

In order to do this I define an interface "Code" like this:

public interface Code {
    public int Add (int a, int b);
    public int Multiply (int a, int b);

}

and then a decorator abstract class "CodeExtention "

public abstract class CodeExtention implements Code{
    protected Code extendedCode;

    public CodeExtention(Code extendedCode) {
        this.extendedCode = extendedCode;
    }

    @Override
    public  int Multiply(int a, int b){
        return extendedCode.Multiply(a, b);
    }

    @Override
    public  int Add(int a, int b){
        return extendedCode.Add(a, b);
    }

}

Now I define a concert class "AVG" extended from my abstract class like this :

public class AVG extends CodeExtention{
    public AVG(Code extendedCode) {
        super(extendedCode);
    }

    public int AVGcalculator (int a, int b){
        return (a+b)/2;    
    }

    @Override
    public int Add(int a, int b) {
        return super.Add(a, b); 
    }

    @Override
    public int Multiply(int a, int b) {
        return super.Multiply(a, b); 
    }

}

Now I expect that my app can calculate the average to do so I have in my main :

    public static void main(String[] args) {
        Code app = new App();
        app = new AVG(app);

    }   
} 

Here I can have :

System.out.println(app.Add(3, 4));
System.out.println(app.Multiply(3, 4));

I still cannot have:

System.out.println(app.AVGcalculator(3, 4));

I don't know what is wrong, or even if I can use this design pattern for my scenario!