Time Limit : 2 Second
Memory Limit : 128 MB
Submission: 62
Solved: 37
Description
N (1 <= N <= 100) cows, conveniently numbered 1..N, are participating
in a programming contest. As we all know, some cows code better
than others. Each cow has a certain constant skill rating that is
unique among the competitors.

two cows. If cow A has a greater skill level than cow B (1 <= A <=
N; 1 <= B <= N; A != B), then cow A will always beat cow B.

Farmer John is trying to rank the cows by skill level. Given a list
the results of M (1 <= M <= 4,500) two-cow rounds, determine the
number of cows whose ranks can be precisely determined from the
results. It is guaranteed that the results of the rounds will not
Input

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

* Lines 2..M+1: Each line contains two space-separated integers that
describe the competitors and results (the first integer, A, is
the winner) of a single round of competition: A and B
Output
* Line 1: A single integer representing the number of cows whose ranks
can be determined
sample input
```5 5
4 3
4 2
3 2
1 2
2 5
```
sample output
`2`
hint
source
USACO JAN08