please solve the constructor issue

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C#

C#4.0

I am using VS2010, and C#4. I have created a Console application in which I have a class that looks like this:

class cl
{
  public int a, b;

  public cl()
  {
  }
  public cl(int x = 10, int y = 20)
  {
    this.a = x;
    this.b = y;
  }
  public void add()
  {
    Console.WriteLine(a+b);
  }
}

I call this from the Main function like this:

class Program
{
  static void Main(string[] args)
  {
    cl obj = new cl(10);
    obj.add();
    Console.ReadKey();
  }
}

I would expect this program to fail to compile since I am passing a single parameter while creating the class object. Can somebody help me in understanding this behaviour?