Xiaohong planted trees around the playground. At the beginning, she planted one tree every 3 meters. After planting 13 trees, she found that the saplings were not enough, so she decided to replant the trees every 4 meters. How many trees were pulled down?
小红在操场周围种树，开始时每隔3米种一棵，种到13棵后，发现树苗不够，于是决定重种，改为每隔4米一棵，这时重种时，不必再拔掉的树有多少棵？
<result>4</result>