13 is missing. "19 watermelons, eat 9 watermelons first, then eat 4 watermelons, how many watermelons are still missing?" This is a math application problem in the first grade of primary school. According to the dry judgment in the question, if you eat nine watermelons first and then four watermelons, you get 9+4= 13, so you are missing one * * * 13 watermelon.