#include <bits/stdc++.h>
using namespace std;
const int RLEN = 1 << 20 | 1;
inline char gc() {
  static char ibuf[RLEN], *ib, *ob;
  (ob == ib) && (ob = (ib = ibuf) + fread(ibuf, 1, RLEN, stdin));
  return (ob == ib) ? EOF : *ib++;
}
inline int read() {
  char ch = getchar();
  int res = 0, f = 1;
  while (!isdigit(ch)) f ^= ch == '-', ch = getchar();
  while (isdigit(ch)) res = (res + (res << 2) << 1) + (ch ^ 48), ch = getchar();
  return f ? res : -res;
}
const int mod = 998244353, g = 3;
inline int add(int a, int b) { return a + b >= mod ? a + b - mod : a + b; }
inline void Add(int &a, int b) { a = add(a, b); }
inline int dec(int a, int b) { return a >= b ? a - b : a - b + mod; }
inline void Dec(int &a, int b) { a = dec(a, b); }
inline int mul(int a, int b) {
  return 1ll * a * b >= mod ? 1ll * a * b % mod : a * b;
}
inline void Mul(int &a, int b) { a = mul(a, b); }
inline int ksm(int a, int b, int res = 1) {
  for (; b; b >>= 1, a = mul(a, a)) (b & 1) ? (res = mul(res, a)) : 0;
  return res;
}
const int N = 105;
int dis[N][N], n, m, st, des, q;
int s[N], d[N], vis[N], pt[N][N], f[N], t, mx[N];
int dfs(int p, int u) {
  if (vis[u] == t) return f[u];
  vis[u] = t;
  int res = -1;
  for (int i = 1; i <= n; i++) {
    if (dis[u][i] == 1 && dis[i][d[p]] + 1 == dis[u][d[p]])
      res = max(res, dfs(p, i));
  }
  if (res == -1) res = 1e9;
  res = min(res, mx[u]);
  return f[u] = res;
}
int main() {
  n = read(), m = read(), st = read(), des = read();
  memset(dis, 127 / 4, sizeof(dis));
  for (int i = 1; i <= n; i++) dis[i][i] = 0;
  for (int i = 1; i <= m; i++) {
    int u = read(), v = read();
    dis[u][v] = 1;
  }
  for (int k = 1; k <= n; k++)
    for (int i = 1; i <= n; i++)
      for (int j = 1; j <= n; j++)
        if (dis[i][j] > dis[i][k] + dis[k][j])
          dis[i][j] = dis[i][k] + dis[k][j];
  q = read();
  for (int i = 1; i <= q; i++) {
    memset(vis, 0, sizeof(vis));
    s[i] = read(), d[i] = read();
    if (dis[s[i]][d[i]] > n) continue;
    for (int j = 1; j <= n; j++)
      if (dis[s[i]][j] + dis[j][d[i]] == dis[s[i]][d[i]]) vis[dis[s[i]][j]]++;
    for (int j = 1; j <= n; j++)
      if (dis[s[i]][j] + dis[j][d[i]] == dis[s[i]][d[i]] &&
          vis[dis[s[i]][j]] == 1)
        pt[i][j] = 1;
  }
  memset(vis, 0, sizeof(vis));
  memset(mx, 127 / 4, sizeof(mx));
  mx[des] = 0;
  bool fg = 1;
  while (fg) {
    fg = 0;
    for (int i = 1; i <= q; i++)
      for (int j = 1; j <= n; j++)
        if (pt[i][j]) {
          t++;
          int now = dfs(i, j) + 1;
          if (now < mx[j]) mx[j] = now, fg = 1;
        }
  }
  if (mx[st] > n)
    puts("-1");
  else
    cout << mx[st] << '\n';
}
