...

src/kr/co/swh/lecture/algorithm/Lv5-2.py

     if st_day<10:
         st_day = "0"+ str(st_day)
     print(str(st_year) + str(st_mon) + str(st_day))
-    st_day = st_day+1
+    st_mon = int(st_mon)
+    st_day = int(st_day)+1
     st_total+=1