#include "bits/stdc++.h"
//#include "atcoder/all"
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
//const bool debug = false;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
vector<int>to[200005];
vector<int>topological_sort(int v) {
	vector<int>h(v);
	rep(i, v) for (auto e : to[i]) ++h[e];
	stack<int>st;
	vector<int>ans;
	rep(i, v) if (h[i] == 0) st.push(i);
	while (!st.empty()) {
		int v = st.top(); st.pop();
		ans.push_back(v);
		for (auto u : to[v]) {
			--h[u];
			if (h[u] == 0) st.push(u);
		}
	}
	return ans;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, k; cin >> n >> k;
	rep(i, k) {
		int r, c; cin >> r >> c; r--; c--;
		to[r].push_back(c);
	}
	auto sortedVs = topological_sort(n);
	if (n != sortedVs.size()) {
		cout << -1 << endl;
		return 0;
	}
	int ans = 1;
	vector<int>d(n);
	rep(i, n) {
		for (auto e : to[sortedVs[i]]) {
			if (chmax(d[e], d[sortedVs[i]] + 1))chmax(ans, d[e] + 1);
		}
	}
	cout << ans << endl;
	return 0;
}