public ActionResult ChangeCheckpoint(int checkpointid)
{
var timeStartnumberModel = (TimeStartnumberModel)Session["TimeStartnumber"];
timeStartnumberModel.Timer.ChangeCheckpoint(checkpointid);
timeStartnumberModel.ChangeCheckpoint(checkpointid);
Session["TimeStartnumber"] = timeStartnumberModel;
return Content(timeStartnumberModel.CheckpointIntermediates[timeStartnumberModel.CurrentCheckpointId].ToListboxvalues());
}