i have some doubt about this problem. it is written in the problem statement that "Vova needs the pipeline to have exactly n pipes with flowing out water."

considering this statement the answer for test case : 21 8 is '-1' . since for k=8 number of pipes can be 1,8,14,19,23,26,28,29 and it is written that pipeline should have exactly n pipes with flowing out water, therefore, 21 cant be possible.

i have run custom test and it is showing me o/p as '4'. am i missing something? please help.