#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, const U &b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, const U &b) {
  if (a < b) a = b;
}
template <class T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T1, class T2>
inline void gn(T1 &x1, T2 &x2) {
  gn(x1), gn(x2);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = 1LL * a * a % m)
    if (b & 1) ans = 1LL * ans * a % m;
  return ans;
}
vector<int> adj[1000100];
int deg[1000100];
int dis[1000100], vst[1000100];
int main() {
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int u, v;
    gn(u, v);
    adj[v].push_back(u);
    deg[u]++;
  }
  int s, t;
  gn(s, t);
  deque<int> q;
  for (int i = 1; i <= n; i++) dis[i] = 0x3f3f3f3f;
  dis[t] = 0;
  q.push_back(t);
  while (!q.empty()) {
    int u = q.front();
    q.pop_front();
    if (vst[u]) continue;
    vst[u] = 1;
    for (int v : adj[u]) {
      deg[v]--;
      if (!deg[v]) {
        if (dis[v] > dis[u]) {
          dis[v] = dis[u];
          q.push_front(v);
        }
      } else if (dis[v] > dis[u] + 1) {
        dis[v] = dis[u] + 1;
        q.push_back(v);
      }
    }
  }
  if (dis[s] < 0x3f3f3f3f)
    cout << dis[s] << endl;
  else
    puts("-1");
  return 0;
}
