#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef int64_t lint;
#define rep(i, n) for(int i=0; i<n; i++)
#define repx(i, l, n) for(int i=l; i<n; i++)
#define all(v) v.begin(), v.end()
#define show(x) cout << #x << ": " << x << endl;
#define list(x) cout << #x << ": " << x << "  ";
#define pb push_back
using vi = vector<lint>;
using vvi = vector<vector<lint>>;
template<class T> inline void vin(vector<T>& v) { rep(i, v.size()) cin >> v.at(i); }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<class T> inline void drop(T x) { cout << x << endl; exit(0); }
template<class T> void vout(vector<T> v) { rep(i, v.size()) { cout << v.at(i) << ' '; } cout << endl; }
constexpr lint LINF = LLONG_MAX/2;
#define mp make_pair
using plint = pair<lint, lint>;
vector<vector<pair<lint, lint>>> G;
vector<vector<pair<lint, lint>>> R;
void make_round_graph(lint N, lint M) {
	lint x, y;
	G.resize(N), R.resize(N);
	rep(i, M) {
		cin >> x >> y;
		x--, y--;
		G[x].push_back(make_pair(1, y));
		R[y].push_back(make_pair(1, x));
	}
}

void dijkstra(vector<lint> &dist) {
	lint a=0, b=0, x, y, N = G.size();
	dist.resize(N, -1);
	priority_queue<plint, vector<plint>, greater<plint>> PQ;
	rep(i, N) {
		if (R[i].size() == 0) {
			dist[i] = 0;
			PQ.push(make_pair(0, i));
		}
	}
	while (!PQ.empty()) {
		tie(x, a) = PQ.top(); PQ.pop();
		if (dist[a] > x) continue;
		for (auto p : G[a]) {
			tie(y, b) = p;
			if (chmax(dist[b], x+y)) PQ.push(make_pair(x+y, b));
		}
	}
}
vi topo_sort() {
	vi res;
	lint N = G.size();
	vi v(N);
	rep(i, N) { for (auto e : G[i]) v[e.second]++; }
	queue<int> Q;
	rep(i, N) { if (!v[i]) Q.push(i); }
	while (!Q.empty()) {
		int x = Q.front(); Q.pop();
		res.push_back(x);
		for (auto e : G[x]) {
			v[e.second]--;
			if (!v[e.second]) Q.push(e.second);
		}
	}
	return res;
}

int main() {
	lint N, K;
	cin >> N >> K;
	make_round_graph(N, K);

	lint a=0, b=0, c=0, x, y, z;
	vi v, w;
	w = topo_sort();
	if (w.size() < N) drop(-1);
	dijkstra(v);
	rep(i, N) {
		chmax(a, v[i]);
	}
	std::cout << a+1 << '\n';
}