#include<bits/stdc++.h>
#define int long long
#define ll long long
#define mod ((int)1e9 + 9)
#define int128 __int128
#define base 23333
#define base2 19260817
#define db double
#define ldb long double
#define eps 1e-8
#define cmpeps 1e-18
#define lowbit(x) (x & -x)
#define un unsigned
#define rep(i,x,y) for (int i = (x); i <= (y); ++i)
#define drep(i,x,y) for (int i = (x); i >= (y); --i)
#define go(i,u) for (int i = head[u]; i; i = edge[i].next)
#define go_(i,u) for (int i = head[u]; ~i; i = edge[i].next)
#define pii pair<int, int>
#define MP make_pair
#define fir first
#define sec second
#define sqr(x) ((x) * (x))
//#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<20,stdin),p1==p2)?EOF:*p1++)
using namespace std;
//char buf[1<<21],*p1=buf,*p2=buf;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<typename T> inline T rnd(T l,T r) {return uniform_int_distribution<T>(l, r)(rng);}
template<typename T> inline void read (T &t) {
	t = 0; char f = 0, ch = getchar(); ldb d = 0.1;
	while (ch > '9' || ch < '0') f |= (ch == '-'), ch = getchar();
	while (ch <= '9' && ch >= '0') t = t * 10 + ch - 48, ch = getchar();
	if (ch == '.') {
		ch = getchar();
		while (ch <= '9' && ch >= '0') t += d * (ch ^ 48), d *= 0.1, ch = getchar();
	}
	t = (f ? -t : t);
}
template <typename T, typename... Args>
	inline void read (T& t, Args&... args) { read(t); read(args...); }
inline void write (int x) {
	if (x >= 10) write(x / 10);
	cout << (ll) (x % 10);
}
const int N = 52, INF = 1e18 + 2;
int n, m, A, B, K;
int val[N], id[N];
int edge[N][N], dis[N][N];
vector<pii > e[N][N];
int f[(1 << 10) + 2][N][N], g[N][N];
int vis[N][N];
inline void add_edge (int u, int v) {
	edge[u][v] = 1; }
struct Node {
	int x, y, val;
	inline bool operator < (const Node &a) const {
		return val < a.val; }
};
priority_queue<Node> q;
inline void calc (int S, int x, int y) {
	if (vis[x][y]) return;
	vis[x][y] = 1;
	f[S][x][y] = g[x][y];
	for (auto _ : e[x][y]) {
		int fi = _.fir, se = _.sec;
		if (g[fi][se] < -f[S][x][y])
			g[fi][se] = -f[S][x][y], q.push((Node) {fi, se, g[fi][se]});
		if (!(--dis[fi][se])) calc(S, fi, se);
	}
}
inline void solve () {
	read(n, m, A, B);
	int u, v;
	rep (i, 1, m) {
		read(u, v);
		add_edge(u, v);
	}
	read(K);
	rep (i, 1, n) id[i] = K + 1;
	int x, w;
	rep (i, 1, K) read(x, w), id[x] = i, val[x] = w;
	rep (S, 0, (1 << K) - 1) {
		rep (i, 1, n) rep (j, 1, n) e[i][j].clear(), dis[i][j] = vis[i][j] = 0;
		rep (i, 1, n) if (!((S >> id[i] - 1) & 1)) rep (j, 1, n) if (!((S >> id[j] - 1) & 1)) {
			g[i][j] = -INF;
			rep (k, 1, n) if (edge[i][k]) {
				if ((S >> id[k] - 1) & 1)
					g[i][j] = max(g[i][j], val[k] - f[S ^ (1 << id[k] - 1)][j][k]);
				else e[j][k].emplace_back(MP(i, j)), ++dis[i][j];
			}
		}
		rep (i, 1, n) rep (j, 1, n) if (!dis[i][j])
			calc(S, i, j);
		while (!q.empty()) q.pop();
		rep (i, 1, n) rep (j, 1, n) if (!vis[i][j])
			q.push((Node) {i, j, g[i][j]});
		while (!q.empty()) {
			Node tp = q.top();
			int x = q.top().x, y = q.top().y;
			q.pop();
			if (g[x][y] <= 0) break;
			calc(S, x, y); 
		}
	}
	printf("%lld\n", f[(1 << K) - 1][A][B]);
}
signed main () {
	//freo();
	int _ = 1;
	//read(_);
	while (_--) solve();
return 0;
}