Xiao Ming planted trees around the playground. At first, he planted one tree every 3 meters. After planting the ninth tree, he found that the saplings were not enough, so he decided to replant the tree every 4 meters. A tree doesn't have to be pulled down?
小明在操场周围种树，开始每隔3米种一棵，种到第9棵后，发现树苗不够了，于是决定重种，改为每隔4米种一棵，重种树时，有几棵树不必拔掉？
<result>3</result>