I failed because of the last move, the problem require 4.a8=Q but 4.b5 seems even more powerfull (not worse anyway).
http://fr.lichess.org/training/21940This problem is bad in the first place because it doesn't accept Rxd2, which is completly fine, but is rated just a bit lower by the engine — +8 vs +7 (sigh).
And yeah, b5 is winning, even better according to the engine.
the problem is rated minus 3,why is it still here?