#include <bits/stdc++.h>
using namespace std;
// #define OPENIOBUF

namespace FastIO
{

class FastIOBase
{
 protected:
#ifdef OPENIOBUF
	static const int BUFSIZE=1<<16;
	char buf[BUFSIZE+1];
	int buf_p=0;
#endif
	FILE *target;
	FastIOBase(FILE *f): target(f){}
	~FastIOBase()=default;
 public:
#ifdef OPENIOBUF
	virtual void flush()=0;
#endif
};

class FastOutput final: public FastIOBase
{
#ifdef OPENIOBUF
 public:
	inline void flush()
	{ fwrite(buf,1,buf_p,target),buf_p=0; }
#endif
 private:
	inline void __putc(char x)
	{
#ifdef OPENIOBUF
		if(buf[buf_p++]=x,buf_p==BUFSIZE) flush();
#else
		putc(x,target);
#endif
	}
	template<typename T>
	inline void __write(T x)
	{
		char stk[64],*top=stk;
		if(x<0) return __putc('-'),__write(-x);
		do *(top++)=x%10,x/=10; while(x);
		for(;top!=stk;__putc(*(--top)+'0'));
	}
 public:
	FastOutput(FILE *f=stdout): FastIOBase(f){}
#ifdef OPENIOBUF
	~FastOutput(){ flush(); }
#endif
	inline FastOutput &operator <<(char x)
	{ return __putc(x),*this; }
	inline FastOutput &operator <<(const char *s)
	{ for(;*s;__putc(*(s++)));return *this; }
	inline FastOutput &operator <<(const string &s)
	{ return (*this)<<s.c_str(); }
	template<typename T>
	inline enable_if_t<is_integral<T>::value,FastOutput&> operator <<(const T &x)
	{ return __write(x),*this; }
	template<typename ...T>
	inline void writesp(const T &...x)
	{ initializer_list<int>{(this->operator<<(x),__putc(' '),0)...}; }
	template<typename ...T>
	inline void writeln(const T &...x)
	{ initializer_list<int>{(this->operator<<(x),__putc('\n'),0)...}; }
	template<typename Iter>
	inline void writesp(Iter begin,Iter end)
	{ while(begin!=end) (*this)<<*(begin++)<<' '; }
	template<typename Iter>
	inline void writeln(Iter begin,Iter end)
	{ while(begin!=end) (*this)<<*(begin++)<<'\n'; }
}qout;

class FastInput final: public FastIOBase
{
#ifdef OPENIOBUF
 public:
	inline void flush()
	{ buf[fread(buf,1,BUFSIZE,target)]=EOF,buf_p=0; }
#endif
 private:
	bool __eof;
	inline char __getc()
	{
		if(__eof) return EOF;
#ifdef OPENIOBUF
		if(buf_p==BUFSIZE) flush();
		char ch=buf[buf_p++];
#else
		char ch=getc(target);
#endif
		return ~ch?ch:(__eof=true,EOF);
	}
	inline void __ungetc(char c)
	{
		__eof=false;
#ifdef OPENIOBUF
		buf_p--;
#else
		ungetc(c,target);
#endif
	}
 public:
	FastInput(FILE *f=stdin): FastIOBase(f),__eof(false)
#ifdef OPENIOBUF
	{ buf_p=BUFSIZE; }
	inline bool eof()const { return buf[buf_p]==EOF; }
#else
	{}
	inline bool eof()const { return feof(target); }
#endif
	inline char peek() { return __getc(); }
	explicit inline operator bool()const { return !__eof; }
	inline FastInput &operator >>(char &x)
	{ while(isspace(x=__getc()));return *this; }
	template<typename T>
	inline enable_if_t<is_integral<T>::value,FastInput&> operator >>(T &x)
	{
		char ch,sym=0;x=0;
		while(isspace(ch=__getc()));
		if(__eof) return *this;
		if(ch=='-') sym=1,ch=__getc();
		for(x=0;isdigit(ch);x=(x<<1)+(x<<3)+(ch^48),ch=__getc());
		return __ungetc(ch),sym?x=-x:x,*this;
	}
	inline FastInput &operator >>(char *s)
	{
		while(isspace(*s=__getc()));
		if(__eof) return *this;
		for(;!isspace(*s) && !__eof;*(++s)=__getc());
		return __ungetc(*s),*s='\0',*this;
	}
	inline FastInput &operator >>(string &s)
	{
		char str_buf[(1<<8)+1]={0},*p=str_buf;
		char *const buf_end=str_buf+(1<<8);
		while(isspace(*p=__getc()));
		if(__eof) return *this;
		for(s.clear(),p++;;p=str_buf)
		{
			for(;p!=buf_end && !isspace(*p=__getc()) && !__eof;p++);
			if(p!=buf_end) break;
			s.append(str_buf);
		}
		__ungetc(*p),*p='\0',s.append(str_buf);
		return *this;
	}
	template<typename ...T>
	inline void read(T &...x)
	{ initializer_list<int>{(this->operator>>(x),0)...}; }
	template<typename Iter>
	inline void read(Iter begin,Iter end)
	{ while(begin!=end) (*this)>>*(begin++); }
#undef __ungetc
}qin;

} // namespace FastIO
using FastIO::qin,FastIO::qout;

using LL=long long;
using LD=long double;
using UI=unsigned int;
using ULL=unsigned long long;
constexpr int INF=2e9;

#ifndef DADALZY
#define FILEIO(file) freopen(file".in","r",stdin),freopen(file".out","w",stdout);
#else
#define FILEIO(file) freopen("ex_jewel2.in","r",stdin)
#endif

int n,m,K,A,B,a[35],b[35];
vector<int> g1[35];

int f[1<<10][35][35],g[35][35],deg[35][35];
bool vis[35][35];
vector<pair<int,int>> g2[35][35];
void solve(int S)
{
	priority_queue<pair<int,pair<int,int>>> q;
	function<void(int,int)> upd=[&](int i,int j)->void
	{
		if(vis[i][j]) return;
		f[S][i][j]=g[i][j],vis[i][j]=true;
		for(auto &[x,y]: g2[i][j])
		{
			if(-f[S][i][j]>g[x][y])
				q.emplace(g[x][y]=-f[S][i][j],make_pair(x,y));
			if(!--deg[x][y]) upd(x,y);
		}
	};
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			g[i][j]=-INF,deg[i][j]=0,vis[i][j]=false,g2[i][j].clear();
	for(int i=1;i<=n;i++) if(~S&a[i])
		for(int j=1;j<=n;j++) if(~S&a[j])
			for(auto &v: g1[i])
			{
				if(S&a[v]) g[i][j]=max(g[i][j],b[v]-f[S^a[v]][j][v]);
				else g2[j][v].emplace_back(i,j),deg[i][j]++;
			}
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			if(!deg[i][j]) upd(i,j);
	q=decltype(q){};
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			if(!vis[i][j]) q.emplace(g[i][j],make_pair(i,j));
	while(!q.empty())
	{
		auto [d,p]=q.top();
		auto [i,j]=p;
		q.pop();
		if(vis[i][j]) continue;
		if(g[i][j]>0) upd(i,j);
		else break;
	}
}

int main()
{
	qin>>n>>m>>A>>B;
	for(int i=1,u,v;i<=m;i++) qin>>u>>v,g1[u].push_back(v);
	qin>>K;
	fill(a+1,a+n+1,1<<K);
	for(int i=1,x,y;i<=K;i++) qin>>x>>y,a[x]=(1<<(i-1)),b[x]=y;
	for(int i=0;i<(1<<K);i++) solve(i);
	qout<<f[(1<<K)-1][A][B]<<'\n';
	return 0;
}