#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

std::vector<int> detect_directed_cycle(
    const std::vector<std::vector<int>>& graph) {
  const int n = graph.size();
  std::vector<int> is_visited(n, 0), cycle;
  const std::function<bool(int)> dfs =
      [&graph, &is_visited, &cycle, &dfs](const int ver) -> bool {
        is_visited[ver] = 1;
        cycle.emplace_back(ver);
        for (const int e : graph[ver]) {
          if (is_visited[e] == 1) {
            cycle.erase(cycle.begin(),
                        std::find(cycle.begin(), cycle.end(), e));
            cycle.emplace_back(e);
            return true;
          } else if (is_visited[e] == 0) {
            if (dfs(e)) return true;
          }
        }
        cycle.pop_back();
        is_visited[ver] = 2;
        return false;
      };
  for (int i = 0; i < n; ++i) {
    if (is_visited[i] == 0 && dfs(i)) break;
  }
  return cycle;
}

int main() {
  int n, q; cin >> n >> q;
  vector<int> a(q), b(q); REP(i, q) cin >> a[i] >> b[i], --a[i], --b[i];
  int lb = -1, ub = q;
  while (ub - lb > 1) {
    const int mid = (lb + ub) / 2;
    vector<vector<int>> graph(n);
    REP(i, mid + 1) graph[a[i]].emplace_back(b[i]);
    (detect_directed_cycle(graph).empty() ? lb : ub) = mid;
  }
  cout << (ub == q ? -1 : ub + 1) << '\n';
  return 0;
}