#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int INF = (int)1e9;
const long long LINF = (long long)1e18;
const long double PI = acos((long double)-1);
const long double EPS = 1e-9;
inline long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
inline long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
inline int chkmin(T& a, const T& val) {
  return a > val ? a = val, 1 : 0;
}
template <class T>
inline int chkmax(T& a, const T& val) {
  return a < val ? a = val, 1 : 0;
}
inline long long isqrt(long long k) {
  long long r = sqrt(k) + 1;
  while (r * r > k) r--;
  return r;
}
inline long long icbrt(long long k) {
  long long r = cbrt(k) + 1;
  while (r * r * r > k) r--;
  return r;
}
inline void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
inline void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
const int maxn = 100010;
int n, m;
vector<int> adj[maxn];
int a[maxn];
int d[maxn];
int tin[maxn];
int tms;
int dfs(int u) {
  tin[u] = tms++;
  int res = tin[a[u]];
  for (int i = (0); i < (int((adj[u]).size())); i++) {
    int v = adj[u][i];
    int r = dfs(v);
    if (r <= tin[u] && r != tin[a[u]]) {
      cout << "-1\n";
      exit(0);
    }
    chkmin(res, r);
  }
  return res;
}
void solve() {
  cin >> n >> m;
  for (int i = (0); i < (m); i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    adj[u].push_back(v);
    d[v]++;
  }
  for (int i = (0); i < (n); i++) cin >> a[i], a[i]--;
  for (int i = (0); i < (n); i++)
    if (!d[i]) {
      dfs(i);
    }
  vector<pair<int, int> > res;
  for (int i = (0); i < (n); i++) {
    res.push_back(make_pair(tin[a[i]], a[i]));
  }
  sort((res).begin(), (res).end());
  (res).erase(unique((res).begin(), (res).end()), (res).end());
  cout << int((res).size()) << "\n";
  for (int i = (int((res).size())) - 1; i >= (0); i--)
    cout << res[i].second + 1 << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  solve();
  return 0;
}
