#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n)for (int i = 0; i < int(n); ++i)
#define rrep(i,n)for (int i = int(n)-1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<class T> void chmax(T& a, const T& b) { a = max(a, b); }
template<class T> void chmin(T& a, const T& b) { a = min(a, b); }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

bool check(const VVI& to) {
  const int n = to.size();
  vector<char> d(n);
  vector<char> visited(n);
  rep(i, n) if (!visited[i]) {
    auto dfs = [&](auto&& self, int u) {
      if (d[u]) return true;
      if (visited[u]) return false;
      d[u] = true;
      visited[u] = true;
      for(int v: to[u]) {
        if (self(self, v)) return true;
      }
      d[u] = false;
      return false;
    };
    if (dfs(dfs, i)) return true;
  }
  return false;
}

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, q;
  cin >> n >> q;
  VI a(q), b(q);
  rep(i, q) cin >> a[i] >> b[i], a[i]--, b[i]--;
  int l = 0, r = q + 1;
  VVI to(n);
  while(r - l > 1) {
    int c = (l + r) / 2;
    rep(i, n) to[i].resize(0);
    rep(i, c) to[a[i]].emplace_back(b[i]);
    if (check(to)) r = c;
    else l = c;
  }
  if (r == q + 1) r = -1;
  cout << r << '\n';
}