#include <bits/stdc++.h>
using namespace std;
int read() {
  char ch = getchar();
  int x = 0, op = 1;
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') op = -1;
  for (; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
  return x * op;
}
int n, m, cnt, head[100005], vis[100005], p[100005];
bool ok[100005];
struct edge {
  int to, nxt;
} e[100005 * 5];
void adde(int x, int y) {
  e[++cnt].to = y;
  e[cnt].nxt = head[x];
  head[x] = cnt;
}
void ins(int x, int y) { adde(x, y); }
long double ithinkcqzispoisonous() {
  return (long double)clock() / CLOCKS_PER_SEC;
}
bool dfs(int u, int ban) {
  if (u == ban) return 0;
  vis[u] = 1;
  for (int i = head[u], v; i; i = e[i].nxt) {
    v = e[i].to;
    if (!vis[v]) {
      if (dfs(v, ban)) return 1;
    } else if (vis[v] == 1) {
      for (int j = (1); j <= (n); j++)
        if (vis[j] != 1) ok[j] = 0;
      return 1;
    }
  }
  vis[u] = 2;
  return 0;
}
bool check(int x) {
  for (int i = (1); i <= (n); i++) vis[i] = 0;
  for (int i = (1); i <= (n); i++)
    if (!vis[i])
      if (dfs(i, x)) return 0;
  return 1;
}
int main() {
  srand(time(NULL));
  n = read();
  m = read();
  for (int i = (1); i <= (m); i++) {
    int x = read(), y = read();
    ins(x, y);
  }
  for (int i = (1); i <= (n); i++) ok[p[i] = i] = 1;
  random_shuffle(&p[1], &p[n + 1]);
  for (int i = (1); i <= (n); i++) {
    if (ithinkcqzispoisonous() > 0.95) break;
    int x = p[i];
    if (ok[x])
      if (check(x)) {
        printf("%d\n", x);
        exit(0);
      }
  }
  puts("-1");
  return 0;
}
