Chef and Class Bunk

Chef’s school has a hectic timetable. There are $N$ periods in a day. In the $i^{th}$ period, Chef is supposed to study subject $a_i$. Since all the subjects are not related to cooking, Chef hates some subjects. Formally, given two integers $x$ and $y$, Chef hates a subject if for that subject $a_i$, $x$ $\leq$ $a_i$ $\leq$ $y$. Find the number of periods Chef has to attend, if he bunks all th