P2419 [USACO08JAN] Cow Contest S

$ 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. 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.

第一行两个用空格隔开的整数 $N, M$。 第 $2\sim M + 1$ 行，每行为两个用空格隔开的整数 $A, B$ ，描述了参加某一轮比赛的奶牛的编号，以及结果（每行的第一个数的奶牛为**胜者**）。

输出一行一个整数，表示排名可以确定的奶牛的数目。

样例解释： 编号为 $2$ 的奶牛输给了编号为 $1, 3, 4$ 的奶牛，也就是说她的水平比这 $3$ 头奶牛都差。而编号为 $5$ 的奶牛又输在了她的手下，也就是说，她的水平比编号为 $5$ 的奶牛强一些。于是，编号为 $2$ 的奶牛的排名必然为第 $4$，编号为 $5$ 的奶牛的水平必然最差。其他 $3$ 头奶牛的排名仍无法确定。