C# isnt correctly subtracting my values : i have a set value and a user entry text box, where i have it set to only numeric entry's, followed by converting it to an int, which all works, but when i run the code and put in a number smaller then the set value it doesn't work out properly

TL:DR

(set value 20, user entered value 12, code returns -4)

    private void LSum_Load(object sender, EventArgs e)
        {
            PassengersTXT.Text = LEFcPassanger.sendtext;
            DepartDateTXT.Text = LutonDepart.sendtext;
            DepTimeTXT.Text = LutonSat.sendtext;
            TravelTypeTXT.Text = LEClass.sendtext;
            DepartTXT.Text = Form2.sendtext;
            ArriveTXT.Text = LArrive.sendtext;

        }
    if( TravelTypeTXT.Text == "First Class")
            {
                Seating.FirstClass -= Convert.ToInt32(PassengersTXT.Text);
                if (Seating.FirstClass <= 0)
                {
                    MessageBox.Show("Not Enough Seats in First Class", "ERROR!",
                    MessageBoxButtons.OK, MessageBoxIcon.Error);
                    LEFcPassanger form = new LEFcPassanger();
                    form.Show();
                    this.Close();
                }
                else
                {
                    Seating.FirstClass -= Convert.ToInt32(PassengersTXT.Text);
                    AppFin frm = new AppFin();
                    frm.Show();
                    this.Close();
                }


    //seperate class
    class Seating
    {
        public static int FirstClass = 20;
        public static int BusinessClass = 36;
        public static int EconomyClass = 84;
        public static int TotalSeats = 140;

    }

