Overload resolution of virtual methods

Consider the code

public class Base
{
   public virtual int Add(int a,int b)
   {
      return a+b;
   }
}

public class Derived:Base
{
   public override int Add(int a,int b)
   {
      return a+b;
   }

   public int Add(float a,float b)
   {
      return (Int32)(a + b);
   }
}

If I create an instance of Derived class and call Add with parameters of type int why it is calling the Add method with float parameters

Derived obj =new Derived()
obj.Add(3,5)

// why this is calling 
Add(float a,float b)

Why it is not calling the more specific method?