Update main.py

main.py

@@ -102,6 +102,10 @@ print(f"\nSelected {len(selected)} file(s):")
 for s in selected:
     print("  ", s)
 
+# enforce minimum 2 files if --all was used
+if args.all and len(selected) < 2:
+    print("⚠️ --all requires at least 2 .m4b files to merge.")
+    sys.exit(1)
 # final output name
 if args.output:
     final_name = args.output.strip()