True and False Paper

Alice wrote an exam containing $N$ true or false questions (i.e. questions whose answer is either true or false). Each question is worth $1$ mark and there is no negative marking in the examination. Alice scored $K$ marks out of $N$. Bob wrote the same exam but he marked each and every question as the opposite of what Alice did, i.e, for whichever questions Alice marked `true`, Bob marked `false`