문제1520--Popular Cows

### 1520: Popular Cows

실행시간 제한: 2 Sec  메모리사용 제한: 64 MB
제출: 41  통과: 12
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#### 문제 설명

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.

#### 입력 설명

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

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

#### 출력 설명

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

#### 입력 예시 Copy

3 3
1 2
2 1
2 3


#### 출력 예시 Copy

1