问do while循环在2个if语句上进入

EN

static void Main(string[] args)
{
    const decimal fifty = 7.25M;
    const decimal hundred = 15.75M;
    const decimal overhundred = 21.50M;
    decimal sumfifty = 0.0M;
    decimal sumhundred = 0.00M;
    decimal sumoverhundred = 0.00M;
    decimal sentinel;

    WriteLine("\n*** PROBLEM 4 ***\n");


    do
    {
        Write("How many pieces? ");
        Write("Enter any whole number or -999 to quit: ");
        stenter code herering input = ReadLine();
        sentinel = Convert.ToDecimal(input);

        sumfifty = sentinel * fifty;
        sumhundred = sentinel * hundred;
        sumoverhundred = sentinel * overhundred;

        if (sentinel >= -998 && sentinel <= 0)
        {
            WriteLine("At 0 pieces at $0.00 per piece, you've earned $0.00.");
        }
        if (sentinel >= 1 && sentinel <= 49)
        {
            WriteLine("At {0} pieces at {1} per piece, you've earned {2}.\n", sentinel, fifty, sumfifty);
        }
        else if (sentinel >= 50 && sentinel <= 100)
        {
                WriteLine("At {0} pieces at {1} per piece, you've earned {2}.\n", sentinel, hundred, sumhundred);
        }
        else
        {
            WriteLine("At {0} pieces at {1} per piece, you've earned {2}.\n", sentinel, overhundred, sumoverhundred);
        }

    } while (sentinel != -999);

    WriteLine("\nThanks for using our system.");
    Write("Press any key to continue...");
    ReadKey();
}