from collections import defaultdict
N,M = map(int,input().split())
D = defaultdict(int)

for i in range(M):
    a,b = map(int,input().split())
    D[a] -= 1
    D[b] += 10**10
 
ans = 0   
for v in D.values():
    if v < 0:
        ans += 1
        
print(ans)