6942: POJ3660 - Cow Contest
[Creator : ]
Description
$N\ (1 ≤ N ≤ 100)$ cows, conveniently numbered $1 \sim N$, 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.
The contest is conducted in several head-to-head rounds, each between 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 be contradictory.
有若干只牛，给出它们之间的一些胜负关系，试求有多少只牛的排名无法确定。
The contest is conducted in several head-to-head rounds, each between 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 be contradictory.
有若干只牛，给出它们之间的一些胜负关系，试求有多少只牛的排名无法确定。
Input
Line $1$: Two space-separated integers: $N, M$
Lines $2 \sim M+1$: 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 $B$
Lines $2 \sim M+1$: 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 1 Input
5 5
4 3
4 2
3 2
1 2
2 5
Sample 1 Output
2