Uncle Li raises a group of cows on the grass. The grass grows evenly every day. If he buys 3 more cows, he will eat the grass 2 days earlier. If he sells 3 cows, he will eat the grass 4 days later. How many days will it take for the original herd of cattle to eat up the grassland.
李大爷在草地上放养一群牛，草地每天均匀生长，如果他再买进3头牛，则会提前2天将草吃完，如果他卖出3头牛，则会推迟4天才能将草地吃完，那么这片草地放养原来那群牛，会用多少天将草地吃完．
<result>8</result>