#include <bits/stdc++.h>
using namespace std;
const int N = 500005;
int cnt, vis[N], n, m, u, v, a[N], tried[N], head[N];
struct node {
  int to, next;
} num[N * 2];
void add(int x, int y) {
  num[++cnt].to = y;
  num[cnt].next = head[x];
  head[x] = cnt;
}
int dfs(int x, int ban) {
  if (x == ban) return 0;
  vis[x] = 1;
  for (int i = head[x]; i; i = num[i].next)
    if (!vis[num[i].to]) {
      if (dfs(num[i].to, ban)) return 1;
    } else if (vis[num[i].to] == 1) {
      for (int j = 1; j <= n; j++)
        if (vis[j] != 1) tried[j] = 1;
      return 1;
    }
  vis[x] = 2;
  return 0;
}
bool check(int ban) {
  for (int i = 1; i <= n; i++) vis[i] = 0;
  for (int i = 1; i <= n; i++)
    if (!vis[i])
      if (dfs(i, ban)) return 0;
  return 1;
}
int main() {
  srand(time(NULL));
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) scanf("%d%d", &u, &v), add(u, v);
  for (int i = 1; i <= n; i++) a[i] = i;
  random_shuffle(a + 1, a + n + 1);
  for (int i = 1; i <= n; i++) {
    if ((double)clock() / CLOCKS_PER_SEC > 0.95) break;
    if (!tried[a[i]])
      if (check(a[i])) return printf("%d\n", a[i]), 0;
  }
  puts("-1");
  return 0;
}
