#include "bits/stdc++.h"
#include <random>
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define SZ(x) ((lint)(x).size())
#define FOR(i, begin, end) for(lint i=(begin),i##_end_=(end);i<i##_end_;++i)
#define IFOR(i, begin, end) for(lint i=(end)-1,i##_begin_=(begin);i>=i##_begin_;--i)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define endk '\n'
using namespace std; typedef unsigned long long _ulong; typedef long long int lint; typedef long double ld; typedef pair<lint, lint> plint; typedef pair<ld, ld> pld;
struct fast_ios { fast_ios() { cin.tie(nullptr), ios::sync_with_stdio(false), cout << fixed << setprecision(20); }; } fast_ios_;
template<class T> auto add = [](T a, T b) -> T { return a + b; };
template<class T> auto f_max = [](T a, T b) -> T { return max(a, b); };
template<class T> auto f_min = [](T a, T b) -> T { return min(a, b); };
template<class T> using V = vector<T>;
using Vl = V<lint>; using VVl = V<Vl>;
template< typename T > ostream& operator<<(ostream& os, const vector< T >& v) {
	for (int i = 0; i < (int)v.size(); i++) os << v[i] << (i + 1 != v.size() ? " " : "");
	return os;
}
template< typename T >istream& operator>>(istream& is, vector< T >& v) {
	for (T& in : v) is >> in;
	return is;
}
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
template <class T>
T div_floor(T a, T b) {
	if (b < 0) a *= -1, b *= -1;
	return a >= 0 ? a / b : (a + 1) / b - 1;
}
template <class T>
T div_ceil(T a, T b) {
	if (b < 0) a *= -1, b *= -1;
	return a > 0 ? (a - 1) / b + 1 : a / b;
}
template <class F> struct rec {
	F f;
	rec(F&& f_) : f(std::forward<F>(f_)) {}
	template <class... Args> auto operator()(Args &&... args) const {
		return f(*this, std::forward<Args>(args)...);
	}
};
lint gcd(lint a, lint b) { if (b == 0) return a; else return gcd(b, a % b); }
lint digit(lint a) { return (lint)log10(a); }
lint e_dist(plint a, plint b) { return abs(a.first - b.first) * abs(a.first - b.first) + abs(a.second - b.second) * abs(a.second - b.second); }
lint m_dist(plint a, plint b) { return abs(a.first - b.first) + abs(a.second - b.second); }
bool check_overflow(lint a, lint b, lint limit) { if (b == 0) return false; return a > limit / b; } // a * b > c => true
void Worshall_Floyd(VVl& g) { REP(k, SZ(g)) REP(i, SZ(g)) REP(j, SZ(g)) chmin(g[i][j], g[i][k] + g[k][j]); }
const lint MOD1000000007 = 1000000007, MOD998244353 = 998244353, INF = 7e18;
lint dx[8] = { -1, 1, 0, 0, 1, -1, 1, -1 }, dy[8] = { 0, 0, 1, -1, -1, -1, 1, 1 };
bool YN(bool flag) { cout << (flag ? "YES" : "NO") << endk; return flag; } bool yn(bool flag) { cout << (flag ? "Yes" : "No") << endl; return flag; }
struct Edge {
	lint from, to;
	string cost;
	Edge() {

	}
	Edge(lint u, lint v, string c) {
		cost = c;
		from = u;
		to = v;
	}
	bool operator<(const Edge& e) const {
		if (SZ(cost) != SZ(e.cost)) return SZ(cost) < SZ(e.cost);
		else return cost < e.cost;
	}
};
struct WeightedEdge {
	lint to;
	lint cost;
	WeightedEdge(lint v, lint c) {
		to = v;
		cost = c;
	}
	bool operator<(const WeightedEdge& e) const {
		return cost < e.cost;
	}
};
using WeightedGraph = V<V<WeightedEdge>>;
typedef pair<plint, lint> tlint;
typedef pair<plint, plint> qlint;
typedef pair<lint, string> valstr;

struct Topological_Sort {
public:

	Topological_Sort(int _n) : G(_n, Vl()), indegree(_n, 0), N(_n) {}

	void add_edge(int u, int v) {
		G[u].push_back(v);
		indegree[v]++;
	}

	Vl get() {
		Vl sorted_vertices;
		Vl tmp_indegree = indegree;
		queue<int> que;
		REP(i, N) {
			if (tmp_indegree[i] == 0) que.push(i);
		}

		while (!que.empty()) {
			int u = que.front(); que.pop();
			REP(i, SZ(G[u])) {
				int v = G[u][i];
				tmp_indegree[v]--;
				if (tmp_indegree[v] == 0) que.push(v);
			}
			sorted_vertices.push_back(u);
		}

		return sorted_vertices;
	}
private:
	VVl G;
	Vl indegree;
	int N;
};

int main() {
	lint N, K;
	cin >> N >> K;
	VVl to(N);
	Topological_Sort topo(N);
	REP(i, K) {
		lint u, v;
		cin >> u >> v; u--; v--;
		to[u].push_back(v);
		topo.add_edge(u, v);
	}
	auto v = topo.get();
	if (SZ(v) != N) {
		cout << -1 << endk;
	}
	else {
		Vl dp(N, 1);
		REP(i, N) {
			for (lint nxt : to[v[i]]) {
				chmax(dp[nxt], dp[v[i]] + 1);
			}
		}
		cout << *max_element(ALL(dp)) << endk;
	}
}