[USACO08JAN]牛大赛Cow Contest

题目背景

[Usaco2008 Jan]

题目描述

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. FJ的N(1 <= N <= 100)头奶牛们最近参加了场程序设计竞赛:)。在赛场上,奶牛们按1..N依次编号。每头奶牛的编程能力不尽相同,并且没有哪两头奶牛的水平不相上下,也就是说,奶牛们的编程能力有明确的排名。 整个比赛被分成了若干轮,每一轮是两头指定编号的奶牛的对决。如果编号为A的奶牛的编程能力强于编号为B的奶牛(1 <= A <= N; 1 <= B <= N; A != B) ,那么她们的对决中,编号为A的奶牛总是能胜出。 FJ想知道奶牛们编程能力的具体排名,于是他找来了奶牛们所有 M(1 <= M <= 4,500)轮比赛的结果,希望你能根据这些信息,推断出尽可能多的奶牛的编程能力排名。比赛结果保证不会自相矛盾。

输入输出格式

输入格式


第1行: 2个用空格隔开的整数:N 和 M 第2..M+1行: 每行为2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(编号为A,即为每行的第一个数的奶牛为 胜者)

输出格式


第1行: 输出1个整数,表示排名可以确定的奶牛的数目

输入输出样例

输入样例 #1

5 5
4 3
4 2
3 2
1 2
2 5

输出样例 #1

2

说明

输出说明: 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。