There is a snail who wants to climb a 20-meter-high tree. It can climb 4.35 meters during the day, but every night when it falls asleep, it slides down another 1.35 meters. How long does it take for the snail to reach the top of the tree?
有一只蜗牛要爬到一棵高20米的树上，它白天能爬4.35米，但是每到晚上它睡着时，又下滑1.35米．这只蜗牛第几天才能爬到树顶？
<result>7</result>