N, M = map(int, input().split())

par = [0] * N
rank = [0] * N

def init(n):
    for i in range(n):
        par[i] = i
        rank[i] = 0
        
def find(x):
    if par[x] == x:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
    
def unite(x, y):
    x = find(x)
    y = find(y)
    if x == y:
        return
    if rank[x] < rank[y]:
        par[x] = y
    else:
        par[y] = x
        if rank[x] == rank[y]:
            rank[x] += 1

def same(x, y):
    return find(x) == find(y)

init(N)
D = dict()
E = dict()

for _ in range(N):
    b, c = map(int, input().split())
    E[c-1] = 0
    if (b-1) not in D:
        D[b-1] = c-1
    else:
        unite(D[b-1], c-1)
        
F = set([find(i) for i in par if i in E])

print(len(D) - len(F))