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문제

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.

입력

  • 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

출력

  • Line 1: A single integer representing the number of cows whose ranks can be determined

예제 입력

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

예제 출력

2

힌트