#include <iostream>

#include <vector>

using namespace std;

typedef struct marble {

bool visited;

vector<int> next_marbles;

vector<int> previous_marbles;

};

vector<marble> Marbles;

int N, M;

int next_dfs(int n) {

int count = 1;

int i;

Marbles[n].visited = true;

for (i = 0; i < Marbles[n].next_marbles.size(); i++) {

if (!Marbles[Marbles[n].next_marbles[i]-1].visited) { count += next_dfs(Marbles[n].next_marbles[i]- 1); }

}

return count;

}

int previous_dfs(int n) {

int count = 1;

int i;

Marbles[n].visited = true;

for (i = 0; i < Marbles[n].previous_marbles.size(); i++) {

if (!Marbles[Marbles[n].previous_marbles[i] - 1].visited) count += previous_dfs(Marbles[n].previous_marbles[i] - 1);

}

return count;

}

int main()

{

int i, j;

cin >> N >> M;

int ans = 0;

for (i = 0; i < N; i++) {

marble new_marble;

Marbles.push_back(new_marble);

}

for (i = 0; i < M; i++) {

int next, x;

cin >> next >> x;

Marbles[x - 1].next_marbles.push_back(next);

Marbles[next - 1].previous_marbles.push_back(x);

}

for (i = 0; i < N; i++) {

for (j = 0; j < N; j++) {

Marbles[j].visited = false;

}

if (next_dfs(i) > (N + 1) / 2) ans += 1;

else {

for (j = 0; j < N; j++) {

Marbles[j].visited = false;

}

if (previous_dfs(i) > (N + 1) / 2) ans += 1;

}

}

cout << ans;

}