import sys
input = sys.stdin.readline
line = input().split()
if not line:
    sys.exit()
n, m = map(int, line)
g = [[] for _ in range(n + 1)]
gr = [[] for _ in range(n + 1)]
for _ in range(m):
    u, v = map(int, input().split())
    g[u].append(v)
    gr[v].append(u)
used = [0] * (n + 1)
order = []
head = [0] * (n + 1)
for i in range(1, n + 1):
    if not used[i]:
        stack = [i]
        used[i] = 1
        while stack:
            v = stack[-1]
            if head[v] < len(g[v]):
                to = g[v][head[v]]
                head[v] += 1
                if not used[to]:
                    used[to] = 1
                    stack.append(to)
            else:
                stack.pop()
                order.append(v)
used = [0] * (n + 1)
ans = 0
for i in reversed(order):
    if not used[i]:
        stack = [i]
        used[i] = 1
        size = 0
        while stack:
            v = stack.pop()
            size += 1
            for to in gr[v]:
                if not used[to]:
                    used[to] = 1
                    stack.append(to)
        ans += min(size, 2)
print(ans)