#include <bits/stdc++.h>
using namespace std;
const int maxn = 111111;
const int INF = 0x3f3f3f3f;
int vis[maxn];
bool selected[maxn];
int n, m;
vector<int> adj[maxn];
bool test[maxn];
double elapsed() { return clock() / (double)CLOCKS_PER_SEC; }
bool dfs(int v) {
  if (selected[v] == true) return false;
  vis[v] = -1;
  for (auto w : adj[v]) {
    if (vis[w] == 0) {
      if (dfs(w)) return true;
    } else if (vis[w] == -1) {
      for (int i = 1; i <= n; i++) {
        if (vis[i] != -1) test[i] = true;
      }
      return true;
    }
  }
  vis[v] = 1;
  return false;
}
bool check() {
  memset(vis, 0, sizeof vis);
  for (int i = 1; i <= n; i++) {
    if (!vis[i] && dfs(i)) return false;
  }
  return true;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 0; i < m; ++i) {
    int u, v;
    scanf("%d%d", &u, &v);
    adj[u].push_back(v);
  }
  vector<int> emp;
  for (int i = 0; i < n; ++i) emp.push_back(i + 1);
  random_shuffle(emp.begin(), emp.end());
  for (int i = 0; elapsed() <= 0.95 and i < emp.size(); i++) {
    selected[emp[i]] = true;
    if (!test[emp[i]] && check()) {
      printf("%d\n", emp[i]);
      return 0;
    }
    selected[emp[i]] = false;
  }
  puts("-1");
}
