#include <iostream>
#include <queue>
#include <vector>
#include <algorithm>

using namespace std;

int n, m;
long long ans[5005] = { 0 };
vector<int> spe[5005];//spe[i][0]表示其入度，spe[i]中其它项对应其它生物
queue<int>q;

int main() {
	ios::sync_with_stdio(false);
	cin >> n >> m;
	for (int i = 1; i <= n; i++)
		spe[i].push_back(0);//刚开始入度默认都为0
	for (int i = 1; i <= m; i++) {
		int uu, vv;
		cin >> uu >> vv;
		spe[vv][0]++;
		spe[uu].push_back(vv);
	}
	for (int i = 1; i <= n; i++)
		if (spe[i][0] == 0) {
			q.push(i);
			ans[i] = 1;
		}
	while (q.empty() == false) {
		int fr = q.front();
		q.pop();
		while(spe[fr].size()>1) {
			int nxt = spe[fr].back();
			spe[fr].pop_back();
			spe[nxt][0]--;
			ans[nxt] = max(ans[nxt], ans[fr] + 1)%80112002;
			if (spe[nxt][0] == 0)
				q.push(nxt);
		}
	}
	sort(ans + 1, ans + n);
	cout << ans[n];
	return 0;
}