Yep, the difference from Yalu river to the 38th parallel is ONLY 500 miles. :D
Yes it is all well known how the US military marched all the way through North Korea upto the Chinese border (some even wanted to go into China), but were then met by the might of the Great peoples army of China who chased them all the way back to below the 38 parallel. Boy I bet that was humiliating, and probably high on American minds when they dare not mess with WMD-armed Kim now.