#include <bits/stdc++.h>
#define cs const
#define pb push_back
using namespace std;
using pi = pair <int , int> ; 

void cmn(int &x, int y) {
    if(x > y) x = y; 
}
void cmx(int &x, int y) {
    if(x < y) x = y; 
}

cs int oo = 1e9 + 7;

using ll = long long ;
cs int mod = 998244353; 

cs int N = 32, M = 1024;
int n, m, A, B;
vector <int> e[N];
int k; 
int w[N];
int id[N];
vector <int> f[M];
vector <int> eb[N * N];

int index(int i, int j){
    return i * n + j; 
}

void work(vector <int> & f, vector <int> & b, vector <int> d) {
    f.resize(n * n); 
    for(int i = 0; i < n * n; i ++)
        f[i] = b[i];
    
    vector <bool> vis(n * n);
    priority_queue <pi> q; 
    queue <int> det; // determine 
    for(int i = 0; i < n * n; i++)
        if(f[i] != - oo) det.push(i), vis[i] = 1; 
    while(1) {
        if(q.empty() && det.empty()) break;
        if(!det.empty()) {
            int x = det.front();
            det.pop();
            for(int v : eb[x]) 
            if(!vis[v]) {
                -- d[v];
                cmx(f[v], - f[x]);
                q.push({f[v], v});
                if(d[v] == 0) {
                    det.push(v);
                    vis[v] = 1; 
                }
            }
        } 
        else {
            auto [v, x] = q.top();
            q.pop();
            if(vis[x]) continue;
            if(f[x] > 0) {
                det.push(x);
                vis[x] = 1; 
            }
        }
    }
    for(int i = 0; i < n * n; i++)
        if(!vis[i]) f[i] = 0; 
}
int main() {
    #ifdef zqj
    freopen("1.in","r",stdin);
    #endif
    
    cin >> n >> m >> A >> B; 
    -- A, -- B; 
    for(int i = 0, u, v; i < m; i++) {
        scanf("%d%d", &u, &v);
        -- u, -- v; 
        e[u].pb(v);
    }
    for(int i = 0; i < n; i++) 
        id[i] = -1; 
    cin >> k; 
    for(int i = 0, x, y; i < k; i++) {
        scanf("%d%d", &x, &y);
        -- x; 
        id[x] = i; 
        w[x] = y; 
    }
    vector <int> d (n * n);
    for(int i = 0; i < n; i++) 
    for(int j = 0; j < n; j++) {
        int x = index(i, j);
        for(int v : e[i]) {
            int y = index(j, v);
            // ee[x].pb(y);
            eb[y].pb(x);
            ++ d[x];
        }
    }
    f[0].resize(n * n);
    for(int S = 1; S < (1 << k); S ++) {
        vector <int> b(n * n, - oo);
        for(int i = 0; i < n; i++) 
        for(int j = 0; j < n; j++) {
            int x = index(i, j);
            if(id[i] >= 0 && (S >> id[i] & 1)) {
                b[x] = w[i] + f[S ^ (1 << id[i])][x];
            }
        }
        work(f[S], b, d);
        // cout << "FFF " << S << endl;
        // for(int x : f[S]) cout << x << ' '; cout << endl;
    }
    cout << f[(1 << k) - 1][index(A, B)] << '\n';
    return 0; 
}