func_code_string: #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;   }   