#include "bits/stdc++.h"
#ifdef DEBUG
#if __cplusplus >= 201700
#include "PrettyDebug.hpp"
#else
#define debug(a) cerr<<"(Line "<<__LINE__<<"): "<<#a<<" = "<<a<<'\n'
#endif
#else
#define debug(...) [](auto...){}(__VA_ARGS__)
#endif
#define For(i,a,b) for(int i=a;i<=b;i++)
#define Rev(i,a,b) for(int i=a;i>=b;i--)
#define Fin(file) freopen(file,"r",stdin)
#define Fout(file) freopen(file,"w",stdout)
#define assume(expr) ((!!(expr))||(exit((fprintf(stderr,"Assumption Failed: %s on Line %d\n",#expr,__LINE__),-1)),false))
#define range basic_string_view
using namespace std; typedef long long ll;
signed main(){
    atexit([](){cerr<<"Time = "<<clock()<<" ms"<<endl;});
    int n,m,A,B; cin>>n>>m>>A>>B; vector<vector<int>> to(n),fo(n); For(i,1,m) { int x,y; cin>>x>>y; to[x-1].push_back(y-1); fo[y-1].push_back(x-1); }
    int K; cin>>K; vector<int> val(n); For(i,1,K) { int x,y; cin>>x>>y; val[x-1]=y; }
    vector<vector<vector<int>>> f(1<<n,vector<vector<int>>(n,vector<int>(n,-1e9)));
    For(i,0,n-1) For(j,0,n-1) f[0][i][j]=0;
    For(s,1,(1<<n)-1){
        auto& F=f[s]; vector<vector<int>> out(n,vector<int>(n));
        For(u,0,n-1) For(v,0,n-1) for(int w:to[u]) if(val[w]&&((s>>w)&1)) F[u][v]=max(F[u][v],val[w]-f[s^1<<w][v][w]); else out[u][v]++;
        priority_queue<tuple<int,int,int>> Q; For(u,0,n-1) For(v,0,n-1) Q.emplace(F[u][v],u,v);
        queue<pair<int,int>> q; For(u,0,n-1) For(v,0,n-1) if(!out[u][v]) q.emplace(u,v);
        while(q.size()||Q.size()){
            while(q.size()){
                auto [u,v]=q.front(); q.pop(); assert(!out[u][v]);
                for(int w:fo[v]) if(out[w][u]) { Q.emplace(F[w][u]=max(F[w][u],-F[u][v]),w,u); if(!--out[w][u]) q.emplace(w,u); }
            }
            while(Q.size()){
                auto [_,u,v]=Q.top(); Q.pop(); if(out[u][v]) {
                    if(F[u][v]>0) { out[u][v]=0,q.emplace(u,v); break; } else goto OUT;
                }
            }
        }
        OUT: For(u,0,n-1) For(v,0,n-1) if(out[u][v]) F[u][v]=0;
    }
    cout<<f[(1<<n)-1][A-1][B-1]<<'\n';
    return 0;
}

// START TYPING IF YOU DON'T KNOW WHAT TO DO
// STOP TYPING IF YOU DON'T KNOW WHAT YOU'RE DOING
// CONTINUE, NON-STOPPING, FOR CHARLIEVINNIE

// Started Coding On: October 16 Mon, 10 : 23 : 14