Understanding Inversion of Control and Dependency Injection

I am learning the concept of IoC and DI. I checked a few blogs and below is my understanding:

A tight-coupling example without the use of IoC:

Public Class A
{
     public A(int value1, int value2)
     {
    return Sum(value1, value2);
     }

     private int Sum(int a, int b)
     {
    return a+b;
     }
}     

After IoC:

Public Interface IOperation
{
    int Sum(int a, int b);
}

Public Class A
{ 
     private IOperation operation;

     public A(IOperation operation)
     {
    this.operation = operation;
     }

     public void PerformOperation(B b)
     {
    operation.Sum(b);
     }
}

Public Class B: IOperation
{
   public int Sum(int a, int b)
   {
       return a+b;
   }
}

The method PerformOperation in Class A is wrong. I guess, it is again tightly coupled as parameter is hard coded as B b.

In the above example, where is IoC as I can see only Dependency Injection in the constructor of Class A.

Please clear my understanding.