if the test data is
"3 1
 2
"
what shall  we output?

Read the problem text :
"
It is guaranteed that it is possible to make next move
irrespective to already made moves.
"

you means that all the test data has a correct answer

There can be "no solution"
The problem text say that there always can be a move :)

in your case when 3 - 2 = 1 left
NO MOVE is possible

and the problem  text say that there can always be a move
until the end of game :)