#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;
const ll oo = 1e16 + 10;
ll dp[mxN][mxN][1<<10];
int where[10];
ll val[10];
bitset<1<<10> vis[mxN][mxN];
vi rev[mxN],adj[mxN];
struct el {
    
    int a,b,msk;
    ll v;
    bool operator<(const el& o) const {
        if(msk!=o.msk) return msk>o.msk;
        return v<o.v;
    }
};
queue<el> q[1<<10];
void process(int a, int b, int msk, ll v) {
    // escapes[a][b][msk]--;
    if(v>dp[a][b][msk]) {
        dp[a][b][msk]=v;
        // if(dp[a][b][msk]>=0) 
        q[msk].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;
        // if(msk) q.push({i,j,msk,0,1});
        // if(msk==0) escapes[i][j][msk]=1;
    }
    for(int i=0;i<n;++i) for(int j=0;j<n;++j) {
        vector<bool> vis(n);
        auto dfs = [&](auto&& self, int at) -> void {
            vis[at]=1;
            for(int to : adj[at]) if(!vis[to]) {
                self(self,to);
            }
        };
        dfs(dfs,i);
        dfs(dfs,j);
        for(int msk=0;msk<(1<<k);++msk) {
            bool good=0;
            for(int o=0;o<k;++o) if((1<<o & msk) and vis[where[o]]) {
                good=1;
            }
            if(!good) process(i,j,msk,0);
        }
    }
    for(int mymsk=0;mymsk<1<<k;++mymsk) {
        auto& myq = q[mymsk];
        while(!myq.empty()) {
            auto e = myq.front(); myq.pop();
            int v = dp[e.a][e.b][e.msk];
            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];
}