#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC optimize("no-stack-protector")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,popcnt,abm,mmx,tune=native")
#pragma GCC optimize("fast-math")
const long long N = 1e6 + 5;
const long double EPS = 1e-12;
const long long INF = 2e18 + 5;
const long long MOD = 1e9 + 7;
const long double PI = 3.14159265358979323;
vector<set<long long> > g;
vector<pair<long long, long long> > a;
vector<long long> ans, u;
void dfs(long long v) {
  u[v] = 1;
  for (auto &i : g[v])
    if (!u[i]) dfs(i);
  ans.push_back(v);
}
void topsort(long long n) {
  ans.clear();
  for (long long i = 0; i < n; i++) {
    if (!u[i]) dfs(i);
  }
  reverse(ans.begin(), ans.end());
}
bool bin(long long n, long long m) {
  for (long long i = 0; i < n; i++) {
    g[i].clear();
    u[i] = 0;
  }
  for (long long i = 0; i < m; i++) {
    g[a[i].first].insert(a[i].second);
  }
  topsort(n);
  for (long long i = 0; i + 1 < ans.size(); i++) {
    if (g[ans[i]].find(ans[i + 1]) == g[ans[i]].end()) return 0;
  }
  return 1;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long ttt = 1;
  for (; ttt; --ttt) {
    long long n, m;
    cin >> n >> m;
    a.resize(m);
    g.resize(n);
    u.resize(n);
    for (long long i = 0; i < m; i++) {
      cin >> a[i].first >> a[i].second;
      a[i].first--;
      a[i].second--;
    }
    if (!bin(n, m)) {
      return cout << "-1\n", 0;
    }
    long long l = 1, r = m;
    while (r - l > 1) {
      long long mid = (l + r) / 2;
      if (mid <= m && bin(n, mid))
        r = mid;
      else
        l = mid;
    }
    if (bin(n, l))
      cout << l << '\n';
    else
      cout << r << '\n';
  }
  return 0;
}
