#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T BM(T p, T e, T M) {
  long long int ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return (T)ret;
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
inline T mdINV(T a, T M) {
  return BM(a, M - 2, M);
}
template <class T>
inline T PW(T p, T e) {
  long long int ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p);
    p = (p * p);
  }
  return (T)ret;
}
template <class T>
string NTS(T Number) {
  stringstream ss;
  ss << Number;
  return ss.str();
}
template <class T>
T stringtonumber(const string &Text) {
  istringstream ss(Text);
  T result;
  return ss >> result ? result : 0;
}
template <class T>
bool ISLEFT(T a, T b, T c) {
  if (((a.first.first - b.first.first) * (b.second.second - c.second.second) -
       (b.first.first - c.first.first) * (a.second.second - b.second.second)) <
      0.0)
    return 1;
  else
    return 0;
}
int n, m;
vector<int> a[200000 + 2];
int ar[200000 + 2];
vector<int> v;
int ans;
int vis[200000 + 2];
int lev[200000 + 2];
int dfs(int u, int l) {
  vis[u] = 1;
  lev[u] = l;
  int num = 200000 + 2;
  for (int i = 0; i < a[u].size(); i++) {
    int first = a[u][i];
    num = min(num, dfs(first, l + 1));
  }
  if (num != lev[ar[u]] && num < lev[u]) {
    ans = -1;
  }
  num = min(num, lev[ar[u]]);
  if (num >= l) v.push_back(u);
  return num;
}
int main() {
  scanf("%d%d", &n, &m);
  int par[n + 2];
  memset(par, 0, sizeof par);
  for (int i = 1; i <= m; i++) {
    int first, second;
    scanf("%d%d", &first, &second);
    a[first].push_back(second);
    par[second]++;
  }
  for (int i = 1; i <= n; i++) scanf("%d", &ar[i]);
  for (int i = 1; i <= n; i++) {
    int first;
    if (!par[i]) {
      first = dfs(i, 1);
    }
  }
  if (ans == -1)
    printf("%d\n", -1);
  else {
    int l = ((int)v.size()) - 1 + 1;
    printf("%d\n", l);
    for (int i = 0; i <= l - 1; i++) {
      printf("%d\n", v[i]);
    }
  }
  return 0;
}
