return value of a function passed to another function as a parameter

When I tried to use the return value of a function which was passed to another function, the script compiler showed an error message like this :

"BCE0051: Operator '+' cannot be used with a left hand side of type 'Object' and a right hand side of type 'int'."

Here is the code :

 function Start ()
 {
     B(A);
 }
 
 function A(aa : int) : int
 {
     return aa+10;
 }
 
 function B(bb : Function)
 {
     Debug.Log(bb(5)+10);
 }
 

which is super simple.

I think the compiler regard the return value of bb(5) as an object. How can I use the return value adequately?