Break out or end a method if a condition is met

If the user enters a number higher than say 600, I would like to run a different class method. I'm start with the AlphaAlphaSeq Class, if the user uses up all the letters, then switch to a between Class and don't run any more code after that. Please the below code:

static void Main(string[] args)
        {

                   NumberSequences n = new NumberSequences();
                   AlphaAlphaSeq a = new AlphaAlphaSeq();

                   Console.WriteLine("uhp" + n.GetNumbersSeq(2));
                   Console.WriteLine(&quo ...