Another little hint if you can't quite make it to the 4th passage in time:
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×Just free up the 3rd passage, don't spend time as described above. Go on to the 4th and then back to the 3rd.
So that is the intended way of doing it?
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×When trying to do them all in order, I ended up exactly one move short of reaching the last passage before it being blocked. I thought something like that must have been done deliberately, before realising that I could easily do what you said above. Seemed like cheating a little, though.