#include <bits/stdc++.h>
#define rep(i,l,r) for(int i=l; i<=r; i++)
#define per(i,r,l) for(int i=r; i>=l; i--)
#define IOS {cin.tie(0);cout.tie(0);ios::sync_with_stdio(0);}
using namespace std;

typedef long long ll;
typedef pair<int,int> P;

const int N = 33;

vector<int> e[N], sta[N], b[N];

int pos[N], w[N], f[(1<<10)+2][33][33], id[N], du[N][N], vis[N][N];
P tmp[1025];

queue <P> q;
multiset<P> ksj;

int main() {
    int n, m, A, B;
    scanf("%d%d%d%d", &n, &m, &A, &B);
    rep (i, 1, m) {
        int u, v;
        scanf("%d%d", &u, &v);
        e[u].push_back(v);
        b[v].push_back(u);

    }
    int k; scanf("%d", &k);
    rep (i, 1, k) {
        scanf("%d%d", &pos[i], &w[i]);
        id[pos[i]] = i;
    }
    int mx = (1<<k) - 1;
    rep (s, 1, mx) {
        sta[__builtin_popcount(s)].push_back(s);
    }
    rep (s, 1, mx) {
        rep(i, 1, n) {
            rep (j, 1, n) {
                f[s][i][j] = -1e9;
            }
        }
    }
    rep (_, 1, k) {
        for (auto s: sta[_]) {
            auto g = f[s];
            rep (i, 1, n) {
                rep (j, 1, n) {
                    int top = 0;
                    for (auto v: e[i]) {
                        int p = id[v];
                        if ((s>>p-1) & 1) {
                            int t = s ^ (1<<p-1);
                            g[i][j] = max(g[i][j], -f[t][j][v] + w[p]);
                            tmp[++top] = P(-f[t][j][v] + w[p], i * n + j - 1);
                        } else {
                            du[i][j]++;
                        }
                    }
                    if (du[i][j] == 0) {
                        vis[i][j] = 1;
                        q.push(P(i, j));
                    } else {
                        while (top) ksj.insert(tmp[top--]);
                    }
                }
            }
            while (1) {
                while (!q.empty()) {
                    auto [i, j] = q.front();
                    q.pop();
                    for (auto v: b[j]) { //previous: (v, i)
                        du[v][i]--;
                        g[v][i] = max(g[v][i], -g[i][j]);
                        if (du[v][i] == 0) {
                            vis[v][i] = 1;
                            q.push(P(v, i));
                        }
                    }
                }
                if (ksj.empty()) break;
                auto [v, t] = *ksj.begin();
                ksj.erase(ksj.begin());
                int i = t/n, j = t%n+1;
                vis[i][j] = 1;
                g[i][j] = v;
                q.push(P(i, j));
            }

            rep (i, 1, n) {
                rep (j, 1, n) {
                    if (!vis[i][j]) g[i][j] = 0;
                    vis[i][j] = du[i][j] = 0;
                }
            }
        }
    }

    cout << f[mx][A][B];
    return 0;
}