#include <cstdio>
#include <memory>
#include <algorithm>
using namespace std;
struct node
{
	int winner;
	int loser;
};
int main()
{
	int n,m,i,count = 0;
	int* a;
	bool changed;
	node* result;
	scanf("%d %d",&n,&m);
	a = new int[n];
	result = new node[m];
	for(i = 0;i < m;i ++)
	{
		scanf("%d %d",&result[i].winner,&result[i].loser);
		result[i].winner--;
		result[i].loser--;
	}
	memset(a,0,n*sizeof(a[0]));
	while(1)
	{
		changed = false;
		for(i = 0;i < m;i ++)
		{
			if(a[result[i].winner] <= a[result[i].loser])
			{
				a[result[i].winner] = a[result[i].loser]+1;
				changed = true;
			}
		}
		if(!changed)
			break;
	}
	sort(a,a+n);
	for(i = 1;i < n;i ++)
	{
		if(a[i] != a[i-1])
			count ++;
		else
			break;
	}
	if(count == n-1)
		count = n;
	printf("%d\n",count);
	delete[] result;
	delete[] a;
	return 0;
}