#include <bits/stdc++.h>
using namespace std;
vector<int> adia[100010];
int n;
int luat[100010];
int skiped[100010];
int tata[100010];
int skip, add;
int dfs(int nod) {
  if (nod == skip) return 1;
  luat[nod] = 1;
  for (auto i : adia[nod]) {
    if (luat[i] == 1) {
      for (int j = nod; j != tata[i]; j = tata[j]) skiped[j]--;
      add++;
      return 0;
    } else if (luat[i] == 0) {
      tata[i] = nod;
      if (!dfs(i)) return 0;
    }
  }
  luat[nod] = 1e9;
  return 1;
}
bool cbin() {
  memset(luat, 0, sizeof luat);
  for (int i = 1; i <= n; i++)
    if (!luat[i] && !dfs(i)) return false;
  return true;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int m, a, b;
  cin >> n >> m;
  while (m--) {
    cin >> a >> b;
    adia[a].push_back(b);
  }
  vector<int> v(n);
  for (int i = 0; i < n; i++) v[i] = i + 1;
  random_shuffle(v.begin(), v.end());
  for (auto i : v) {
    if (1. * clock() / CLOCKS_PER_SEC >= 0.9) {
      cout << "-1\n";
      return 0;
    }
    if (skiped[i] + add > 0) continue;
    skip = i;
    if (cbin()) {
      cout << i << '\n';
      return 0;
    }
  }
  cout << "-1\n";
  return 0;
}
