#include <bits/stdc++.h>
int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
using namespace std;
bool b[100000 + 5];
vector<int> v[100000 + 5];
int n, m;
int s[100000 + 5];
int ans[100000 + 5], cnt = 0;
bool yes = true;
void dfs(int x, int fa) {
  if (fa != 0 && s[x] != x && s[x] != s[fa]) {
    puts("-1");
    yes = false;
  }
  if (s[x] == x) ans[++cnt] = x;
  for (int i = 0; i < v[x].size(); i++) {
    int y = v[x][i];
    if (y == fa) continue;
    dfs(y, x);
  }
}
int main() {
  n = read();
  m = read();
  for (int i = 1; i <= m; i++) {
    int x = read(), y = read();
    v[x].push_back(y);
    b[y] = 1;
  }
  for (int i = 1; i <= n; i++) s[i] = read();
  for (int i = 1; i <= n; i++) {
    if (!b[i]) dfs(i, 0);
  }
  if (yes == true) {
    cout << cnt << endl;
    for (int i = cnt; i >= 1; i--) {
      cout << ans[i] << endl;
    }
  }
  return 0;
}
