fix invalid combine

app.py

@@ -53,12 +53,12 @@ def is_valid(aa: str, bb: str):
     if f"{aa}{bb}" in invalid:
         return False
 
-    if aa == "y":
-        if bb[0] == "y":
+    if bb[0] == "y":
+        if aa[0] == "y":
             return False
-        if bb[0] == "w":
+        if aa == "w":
             return False
-        if bb == "":
+        if aa == "":
             return False
 
     if aa == "a" and bb[0] == "y":