using System;

public class Program

{

    public  static int NumWaterBottles(int numBottles, int numExchange)

    {

        if(numBottles <= 0 || numExchange <= 0)

            return 0;

        var freeBottles = numBottles;

        var resultBottles = numBottles;

        while(freeBottles >= numExchange)

        {

            var newBottles= freeBottles / numExchange;

            resultBottles +=  newBottles;

            freeBottles = freeBottles%numExchange + newBottles;

        }

        return resultBottles;

    }

    public static void Main()

    {

        var result = NumWaterBottles(15,4);

        Console.WriteLine($"result {result}");

    }

}