#include<iostream>
#include<cstring>
using namespace std;
const int maxn = 505;
int n,m,Map[maxn][maxn],vis[maxn];
int min_dfs(int x){
	vis[x]=1;
	int ans=1;
	for(int i=1;i<=n;i++){
		if(Map[x][i]==1&&!vis[i]){
			ans+=min_dfs(i);
		}
	}
	return ans;
}
int max_dfs(int x){
	vis[x]=1;
	int ans=1;
	for(int i=1;i<=n;i++){
		if(Map[x][i]==2&&!vis[i]){
			ans+=max_dfs(i);
		}
	}
	return ans;
}
int main(){
	cin>>n>>m;
	int h=(n+1)>>1;
	while(m--){
		int x,y;
		cin>>x>>y;
		Map[x][y]=1;  //x->y
		Map[y][x]=2;  //y->x
	}
	int cnt=0;
	for(int i=1;i<=n;i++){
		int mins=min_dfs(i);
		memset(vis,0,sizeof(vis));
		int maxs=max_dfs(i);
		memset(vis,0,sizeof(vis));
//		if(mins>=1&&mins<=h&&maxs>=0&&maxs<=h){
//			continue;
//		}else
		if(mins>h||maxs>h){
			cnt++;
		}
	}
	cout<<cnt<<endl;
	return 0;
}