Reduce DLL size in Android Project

HI everyone, I have created a simple project (I am a Unity beginner) for Android but the apk size is around 18 MB! I checked the documentation. One way to decrease the apk size is to check the Editor,log. Then i discovered that imported DLLs have a size of 3.9MB. I only used UnityEngine and Collision packages. So how can I avoid to import this big dlls?