So you can add or subtract exactly five at a time no matter what, right? Level 13 has a total that doesn't divide by five, so it's unwinnable unless I missed a game mechanic.
Actually not - adjacent blank squares or squares already at +3/-3 can nullify this rule you identified (there may be, in some cases, less than 5 interactable squares if there are many blank squares or maxed out squares). But this isn't the case on level 13 :)
Anyway, level 13 can be beaten in 15ish moves (check screen, one click away from win). Try to play around with bringing max or min values on squares (+3, -3):
In general all levels have been generated starting from a blank grid and clicking back and forth, and then giving a generous amount of moves to the player to "reverse" such process.
There is also a secret trick, which costs an hefty amount of moves: bringing all the squares in a + up to 3 or -3, and then bringing them down/up to 0. Costly, but works! ;)
57 days ago (+1)
Ah - didn't know about the max value. The level wraps around all the blank squares, so I had thought it was impossible to do anything other than add or subtract 5 at a time.
(for any readers, there are more levels now, added at the start I think. The old level 13 is level 18)