Popular Cows 
Every cow's dream is to become the most popular cow in the herd. In a herd
of N (1 <= N <= 10,000) cows, you are given up to M (1 <= M <= 50,000)
ordered pairs of the form (A, B) that tell you that cow A thinks that cow B
is popular. Since popularity is transitive, if A thinks B is popular and B
thinks C is popular, then A will also think that C is popular, even if this
is not explicitly specified by an ordered pair in the input. Your task is
to compute the number of cows that are considered popular by every other cow.

PROBLEM NAME: popular

INPUT FORMAT:

* Line 1: Two space-separated integers, N and M.

* Lines 2..M+1: Two space-separated numbers A and B, meaning that cow
        A thinks cow B is popular.

SAMPLE INPUT:

3 3
1 2
2 1
2 3

OUTPUT FORMAT:

* Line 1: A single integer that is the number of cows who are
        considered popular by every other cow.

SAMPLE OUTPUT:

1

OUTPUT DETAILS:

Cow 3 is the only cow of high popularity.