ERRO

eu executo ele normal, mas quando eu coloco os valores e aperto o botão ele aparece um erro

System.InvalidCastException
  HResult=0x80004002Message=Não é possível converter um objeto do tipo 'System.Windows.Forms.TextBox' no tipo 'System.IConvertible'.
  Source=mscorlib
  StackTrace:
   em System.Convert.ToInt32(Object value)
   em Bascara.Form1.btnBasc_Click(Object sender, EventArgs e) em F:\User\Israel\Desktop\VB\Bascara\Bascara\Form1.cs:linha 37
   em System.Windows.Forms.Control.OnClick(EventArgs e)
   em System.Windows.Forms.Button.OnClick(EventArgs e)
   em System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   em System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   em System.Windows.Forms.Control.WndProc(Message& m)
   em System.Windows.Forms.ButtonBase.WndProc(Message& m)
   em System.Windows.Forms.Button.WndProc(Message& m)
   em System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   em System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   em System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   em System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   em System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   em System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   em System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   em System.Windows.Forms.Application.Run(Form mainForm)
   em Bascara.Program.Main() em F:\User\Israel\Desktop\VB\Bascara\Bascara\Program.cs:linha 19