#include <bits/stdc++.h>
using namespace std;
namespace fast_io {
char buf[1 << 12], *p1 = buf, *p2 = buf, sr[1 << 23], z[23], nc;
int C = -1, Z = 0, Bi = 0, ny;
inline char gc() {
  return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1 << 12, stdin), p1 == p2)
             ? EOF
             : *p1++;
}
inline int read() {
  int x = 0;
  ny = 1;
  while (nc = gc(), (nc < 48 || nc > 57) && nc != EOF)
    if (nc == 45) ny = -1;
  Bi = 1;
  if (nc < 0) return nc;
  x = nc - 48;
  while (nc = gc(), 47 < nc && nc < 58 && nc != EOF)
    x = (x << 3) + (x << 1) + (nc ^ 48), Bi++;
  return x * ny;
}
inline double gf() {
  int a = read(), y = ny, b = (nc != '.') ? 0 : read();
  return (b ? a + (double)b / pow(10, Bi) * y : a);
}
inline int gs(char *s) {
  char c, *t = s;
  while (c = gc(), c <= 32)
    ;
  *s++ = c;
  while (c = gc(), c > 32) *s++ = c;
  return s - t;
}
inline void ot() {
  fwrite(sr, 1, C + 1, stdout);
  C = -1;
}
inline void flush() {
  if (C > 1 << 22) ot();
}
template <typename T>
inline void write(T x, char t) {
  int y = 0;
  if (x < 0) y = 1, x = -x;
  while (z[++Z] = x % 10 + 48, x /= 10)
    ;
  if (y) z[++Z] = '-';
  while (sr[++C] = z[Z], --Z)
    ;
  sr[++C] = t;
  flush();
}
inline void write(char *s) {
  int l = strlen(s);
  for (int i = 0; i < l; i++) sr[++C] = *s++;
  sr[++C] = '\n';
  flush();
}
};  // namespace fast_io
using namespace fast_io;
const int N = 3e5 + 5;
int n, m, ans, vis[N], d[N], f[N], q[N];
vector<int> e[N];
struct did {
  int u, v;
} ed[N];
inline void add(int u, int v) { e[u].push_back(v), d[v]++; }
inline void solve(int p, int u, int s) {
  int op = 0;
  for (int v : e[u])
    if (d[v] == 1) {
      op = 1;
      break;
    }
  op ? vis[p] = 1 : f[p] += s;
}
inline void topsort() {
  int l = 0, r = 0;
  for (int i = 1; i <= n; i++)
    if (!d[i]) q[++r] = i;
  while (l < r) {
    int u = q[++l];
    if (l == r) f[u] += n - r;
    if (l + 1 == r) solve(u, q[r], n - r);
    for (int v : e[u])
      if (!(--d[v])) q[++r] = v;
  }
}
int main() {
  n = read(), m = read();
  for (int i = 1, u, v; i <= m; i++)
    ed[i] = {read(), read()}, add(ed[i].u, ed[i].v);
  topsort();
  for (int i = 1; i <= n; i++) e[i].clear();
  for (int i = 1; i <= m; i++) add(ed[i].v, ed[i].u);
  topsort();
  for (int i = 1; i <= n; i++)
    if (!vis[i] && f[i] >= n - 2) ans++;
  write(ans, '\n');
  return ot(), 0;
}
