#include "bits/stdc++.h"
using namespace std;
#define all(x) begin(x),end(x)
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { string sep; for (const T &x : v) os << sep << x, sep = " "; return os; }
#define debug(a) cerr << "(" << #a << ": " << a << ")\n";
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pi;
const int mxN = 30, oo = 2e9 + 20;
int dp[mxN][mxN][1<<10];
int escapes[mxN][mxN][1<<10];
int where[10];
int val[10];
bitset<1<<10> vis[mxN][mxN];
vi rev[mxN],adj[mxN];
struct el {
    
    int a,b,msk;
    int v;
    bool wish=0;
    bool operator<(const el& o) const {
        if(msk!=o.msk) return msk>o.msk;
        if(wish!=o.wish) return wish>o.wish;
        return v<o.v;
    }
};
priority_queue<el> q;
void process(int a, int b, int msk, int v) {
    escapes[a][b][msk]--;
    if(v>dp[a][b][msk]) {
        dp[a][b][msk]=v;
        if(dp[a][b][msk]>=0) q.push({a,b,msk,v});
    }
    if(escapes[a][b][msk]==0) {
        if(dp[a][b][msk]<0) {
            q.push({a,b,msk,dp[a][b][msk]});
        }
    }

}
int main() {
    int n,m,a,b; cin >> n >> m >> a >> b;
    --a,--b;
    while(m--) {
        int u,v; cin >> u >> v;
        --u,--v;
        rev[v].push_back(u);
        adj[u].push_back(v);
    }
    int k; cin >> k;
    for(int i=0;i<k;++i) {
        cin >> where[i]; --where[i];
        cin >> val[i];
    }

    for(int i=0;i<n;++i) for(int j=0;j<n;++j) for(int msk=0;msk<1<<k;++msk) {
        escapes[i][j][msk] = adj[i].size();
        dp[i][j][msk]=-oo;
        q.push({i,j,msk,0,1});
    }
    for(int i=0;i<n;++i) for(int j=0;j<n;++j) {
        process(i,j,0,0);
    }
    while(!q.empty()) {
        auto e = q.top(); q.pop();
        int& v = dp[e.a][e.b][e.msk];
        if(vis[e.a][e.b][e.msk]) continue;
        if(e.wish) {
            if(e.v>v) {
                v=e.v;
            }
        }
        vis[e.a][e.b][e.msk]=1;
        for(auto from : rev[e.b]) {
            process(from,e.a,e.msk, -v);
            for(int j=0;j<k;++j) if(where[j]==e.b and (e.msk & 1<<j)==0) {
                process(from,e.a,e.msk|(1<<j),-v + val[j]);
            }
        }
    }
    cout << dp[a][b][(1<<k)-1];
}