diff --git a/units/en/unit1/additional-readings.mdx b/units/en/unit1/additional-readings.mdx
index b881244..d1f1820 100644
--- a/units/en/unit1/additional-readings.mdx
+++ b/units/en/unit1/additional-readings.mdx
@@ -11,3 +11,4 @@ These are **optional readings** if you want to go deeper.
 ## Gym [[gym]]
 
 - [Getting Started With OpenAI Gym: The Basic Building Blocks](https://blog.paperspace.com/getting-started-with-openai-gym/)
+- [Make your own Gym custom environment](https://www.gymlibrary.dev/content/environment_creation/)