static void Main(string[] args)
        {
            int[] billValues = { 1, 5, 10, 20, 50, 100 };
            int[] myBills = new int[6]; // which bills I have
            int[] paidBills = new int[6]; // what bills I paid
            int[] changeBills = new int[6]; // what bills I got as change

            int amountToPay = 150;

            int amountLeftToPay = amountToPay;

            myBills[0] = 0; //1
            myBills[1] = 0; //5
            myBills[2] = 1; //10
            myBills[3] = 0; //20
            myBills[4] = 0; //50
            myBills[5] = 2; //100

            int Total = (myBills[0] * 1) + (myBills[1] * 5) + (myBills[2] * 10) + (myBills[3] * 20) + (myBills[4] * 50) + (myBills[5] * 100);

            if(amountToPay > Total)
            {
                Console.WriteLine("Not enough money");
                Console.ReadLine();
                return;
            }

            for (int i = billValues.Length - 1; i >= 0; i--) // biggest FIRST
            {
                // basically keep paying with e.g. hundreds as long as the amount
                // left is 100+, etc, then go on to 50's etc

                while (amountLeftToPay > 0 && myBills[i] > 0)
                {
                    amountLeftToPay -= billValues[i];
                    myBills[i]--;
                    paidBills[i]++;
                    Total -= billValues[i];
                }
            }

            int Change = amountLeftToPay *-1;

            Console.WriteLine("1$: " + myBills[0]);
            Console.WriteLine("5$: " + myBills[1]);
            Console.WriteLine("10$: " + myBills[2]);
            Console.WriteLine("20$: " + myBills[3]);
            Console.WriteLine("50$: " + myBills[4]);
            Console.WriteLine("100$: " + myBills[5]);
            Console.WriteLine("Total$: " +Total);
            Console.WriteLine("Change: "+Change);
        }