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.

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.

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

2