shepherd grazes his sheep on an evenly withered meadow. He slaughters one sheep every day at the end of the grazing. If there are 50 sheep at the beginning, then the grass will be eaten in exactly 9 days. If there are 45 sheep at the beginning, then exactly 10 days. After eating this piece of grass, it is now required that this piece of grassland can be supplied for at least 20 days. How many sheep can the shepherd start herding at most?
牧羊人在一片均匀枯萎的草地上放羊，他每天放牧结束后都会宰杀一只羊，如果开始有50只羊，那么恰好9天后吃完这片草，如果开始有45只羊，那么恰好10天吃完这片草，现在要求这片草地要能供应至少20天，牧羊人开始最多能放养多少只羊？
<result>25</result>