Mr. and Mrs. Ali and their son Tulu want to cross the river Rupsha. Mr. Kashem's rowboat is the only means to cross the river. Mr. Kashem will not allow anyone to row his boat and will take only one passenger at a time. Tulu is only a little boy. So he cannot be let alone on the river bank. Which of the following conditions is not part of the successful passage of the Ali's across the river?