4102：Books

Ali’s room is full of books and he has never come up with a good way to arrange them. He just put them on the desk. But sometimes the books became unstable due to the gravity, and some might slide off the desk. He found it very annoying and decided to use a new method to put his books.
He bought some magic clips to stabilize the books. Once he put a book onto the desk, he will check if the book is steady. If it is, then he will use a clip to stabilize it and it will never slide or flip. If not, he will just pick up the book and throw it onto the floor and deal with it later.
But as he began to use the new method, he found it hard to decide whether a book is steady when put it on the desk. So he needs your help to determine that.
Assume that every book has the same thickness.
A book is steady means it will stay unmoved and horizontal even if given a small force.

The input consists several testcases.
The first line contains the number of books N (1 <= N <= 2000).
The following N lines each contains four integers, x₁,y₁,x₂,y₂ (-10³ <= x₁, y₁, x₂, y₂ <= 10³), means that the i-th book is placed with lower-left-corner placed at x₁,y₁ and upper-right-corner placed at x₂,y₂ (if you look down from above). The book’s edge is always paralleled with the desk’s edge.

Print one integer, the number of books that are remained on the desk.