#include <bits/stdc++.h>
const int N = 109;
int a, b, n, m, f[N][N], dp[N][N], ways[N][N], s[N], q[N * N][2], deg[N][N],
    t[N];
bool inq[N][N];
inline int onpath(int x, int s, int t) {
  if (f[s][x] + f[x][t] != f[s][t]) return 0;
  return 1 + (ways[s][x] * ways[x][t] == ways[s][t]);
}
int main() {
  scanf("%d%d%d%d", &n, &m, &a, &b);
  memset(f, 0x3f, sizeof f);
  for (int x, y; m--;) {
    scanf("%d%d", &x, &y);
    f[x][y] = 1;
    ways[x][y] = 1;
  }
  for (int i = 1; i <= n; ++i) f[i][i] = 0, ways[i][i] = 1;
  for (int k = 1; k <= n; ++k)
    for (int i = 1; i <= n; ++i)
      if (i != k)
        for (int j = 1; j <= n; ++j)
          if (i != j && k != j) {
            if (f[i][j] > f[i][k] + f[k][j])
              f[i][j] = f[i][k] + f[k][j], ways[i][j] = ways[i][k] * ways[k][j];
            else if (f[i][j] == f[i][k] + f[k][j])
              ways[i][j] += ways[i][k] * ways[k][j];
          }
  scanf("%d", &m);
  memset(dp, 0x3f, sizeof dp);
  dp[b][0] = 0;
  for (int i = 1; i <= m; ++i) {
    scanf("%d%d", s + i, t + i);
    if (ways[s[i]][t[i]] == 0) --i, --m;
  }
  for (int i = 1; i <= m; ++i) {
    for (int j = 1; j <= n; ++j)
      for (int k = 1; k <= n; ++k)
        if (f[j][k] == 1 && onpath(j, s[i], t[i]) && onpath(k, j, t[i]))
          ++deg[i][j];
  }
  for (int dis = 0; dis <= m; ++dis) {
    int h = 1, T = 0;
    for (int i = 1; i <= n; ++i)
      for (int j = 0; j <= m; ++j)
        if (dp[i][j] == dis) {
          ++T;
          q[T][0] = i;
          q[T][1] = j;
          inq[i][j] = true;
        }
    while (h <= T) {
      int u = q[h][0], v = q[h][1];
      ++h;
      if (onpath(u, s[v], t[v]) == 2) dp[u][0] = std::min(dp[u][0], dis + 1);
      if (v) {
        for (int i = 1; i <= n; ++i)
          if (f[i][u] == 1 && onpath(i, s[v], t[v]) && onpath(u, i, t[v]) &&
              !--deg[v][i]) {
            dp[i][v] = dis;
            if (!inq[i][v]) {
              inq[i][v] = true;
              ++T;
              q[T][0] = i;
              q[T][1] = v;
            }
          }
      } else
        for (int i = 1; i <= m; ++i)
          if (onpath(u, s[i], t[i]) && dp[u][i] > dis) {
            dp[u][i] = dis;
            if (!inq[u][i]) {
              inq[u][i] = true;
              ++T;
              q[T][0] = u;
              q[T][1] = i;
            }
          }
    }
  }
  printf("%d\n", dp[a][0] == 0x3f3f3f3f ? -1 : dp[a][0]);
}
