#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;
#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; }
bool topological_sort(vector<vector<int>>to) {
	vector<int>h(to.size());
	rep(i, to.size()) for (auto e : to[i]) ++h[e];
	stack<int>st;
	vector<int>ans;
	rep(i, to.size()) 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.size() == to.size();
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, q; cin >> n >> q;
	vector<int>a(q), b(q);
	rep(i, q) {
		cin >> a[i] >> b[i]; a[i]--, b[i]--;
	}
	auto f = [&](vector<vector<int>>edge) {
		return topological_sort(edge);
	};
	{
		vector<vector<int>>edge(n);
		rep(i, q)edge[a[i]].push_back(b[i]);
		if (f(edge)) {
			cout << -1 << endl;
			return 0;
		}
	}
	int ng = 0;
	int ok = q;
	while (1 < abs(ok - ng)) {
		int c = (ng + ok) / 2;
		vector<vector<int>>edge(n);
		rep(i, c)edge[a[i]].push_back(b[i]);
		if (!f(edge)) ok = c;
		else ng = c;
	}
	cout << ok << endl;
	return 0;
}