#include <bits/stdc++.h>
const int N = 100005;
using namespace std;
int ban[N], vis[N];
vector<int> e[N];
int tried[N], n, m;
bool dfs(int x) {
  if (ban[x]) return 0;
  vis[x] = 1;
  for (int i = 0; i < e[x].size(); i++) {
    int to = e[x][i];
    if (vis[to] == 0) {
      if (dfs(to)) return 1;
    } else if (vis[to] == 1) {
      for (int i = 1; i <= n; i++)
        if (vis[i] != 1) tried[i] = 1;
      return 1;
    }
  }
  vis[x] = 2;
  return 0;
}
int a[N];
double SZB() { return 1.0 * clock() / CLOCKS_PER_SEC; }
bool check() {
  for (int i = 1; i <= n; i++) vis[i] = 0;
  for (int i = 1; i <= n; i++)
    if (!vis[i])
      if (dfs(i)) return 0;
  return 1;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    int x, y;
    scanf("%d%d", &x, &y);
    e[x].push_back(y);
  }
  for (int i = 1; i <= n; i++) a[i] = i;
  random_shuffle(a + 1, a + n + 1);
  for (int i = 1; SZB() < 0.95 && i <= n; i++) {
    if (tried[a[i]]) continue;
    ban[a[i]] = 1;
    if (check()) {
      printf("%d", a[i]);
      return 0;
    }
    ban[a[i]] = 0;
  }
  puts("-1");
}
