/*
膜拜传奇特级宗师 Afterglow 大神儿
今天在 florr 首页称您为大夏尊贵的大名儿
一股敬佩之情油生然而
您在 florr 为国争光，扬我大夏威名
向您献上最真挚的膜拜！！！
膜拜传奇特级宗师 Afterglow 大神儿，今，一，您，扬。向！
膜拜传奇特级宗师 Afterglow 大神儿，今，一，您，扬。向！
膜拜传奇特级宗师 Afterglow 大神儿，今，一，您，扬。向！
*/
/*
          _____                    _____                     _____                    _____
         /\    \                  /\    \                   /\    \                  /\    \
        /  \    \                /  \    \                 /  \____\                /  \    \
        \   \    \              /    \    \               /   /    /                \   \    \
         \   \    \            /      \    \             /   /    /                  \   \    \
          \   \    \          /   /\   \    \           /   /____/                    \   \    \
           \   \    \        /   /  \   \    \         /    |    |                     \   \    \
            \   \    \      /   /    \   \    \       /     |    |                      \   \    \
             \   \    \    /   /    / \   \    \     /      |    |                       \   \    \
              \   \    \  /   /    /   \   \    \   /       |____|__ _____                \   \    \
_______________\   \____\/   /____/     \   \    \ /   /|            \    \ _______________\   \____\
\                  /    /\   \    \      \   \    \\  / |    _________\____\\                  /    /
 \    ____________/____/  \   \    \      \   \____\\/__|   |    |           \    ____________/____/
  \   \    \               \   \    \     |   |    |    |   |    |            \   \    \
   \   \    \               \   \    \    |   |    |    |   |    |             \   \    \
    \   \    \               \   \    \   |   |____|    |   |    |              \   \    \
     \   \    \               \   \    \  /   /    /    \   |    |               \   \    \
      \   \    \               \   \    \/   /    /      \  |    |                \   \    \
       \   \____\               \   \___/   /    /        \ |    |                 \   \____\
        \  /    /                \         /    /          \|    |                  \  /    /
         \/____/                  \_______/____/            \____|                   \/____/
*/
#define poj
#define zcz
#ifdef poj
//C++
#include<iomanip>
#include<iostream>
//C
#include<cassert>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
//STL
#include<algorithm>
#include<bitset>
#include<functional>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<string>
#include<vector>
//C++11
#if __cplusplus>=201103L
#include<chrono>
#include<random>
#include<unordered_set>
#include<unordered_map>
#endif
#else
#include<bits/stdc++.h>
#endif
using namespace std;
#ifdef zcz
class fastin{
	private:
#ifdef poj
	static const int MAXBF=1<<20;
#else
	const int MAXBF=1<<27;
#endif
	FILE *inf;
	char *inbuf,*inst,*ined;
	inline char _getchar(){
		if(inst==ined)inst=inbuf,ined=inbuf+fread(inbuf,1,MAXBF,inf);
		return inst==ined?EOF:*inst++;
	}
	public:
	fastin(FILE*_inf=stdin){
		inbuf=new char[MAXBF],inf=_inf,inst=inbuf,ined=inbuf;
	}
	~fastin(){delete[]inbuf;}
	template<typename Int> fastin&operator>>(Int &n){
		static char c;
		Int t=1;
		while((c=_getchar())<'0'||c>'9')if(c=='-')t=-1;
		n=(c^48);
		while((c=_getchar())>='0'&&c<='9')n=n*10+(c^48);
		n*=t;
		return *this;
	}
	fastin&operator>>(char&c){
		while((c=_getchar())<'!'||c>'~');
		return *this;
	}
	fastin&operator>>(char*s){
		int t=0;
		static char c;
		while((c=_getchar())==' '||c=='\n');
		s[t++]=c;
		while((c=_getchar())>='!'&&c<='~')s[t++]=c;
		s[t]='\0';
		return *this;
	}
}fi;
class fastout{
	private:
#ifdef poj
	static const int MAXBF=1<<20;
#else
	const int MAXBF=1<<27;
#endif
	FILE *ouf;
	char *oubuf,*oust,*oued;
	inline void _flush(){fwrite(oubuf,1,oued-oust,ouf);oued=oust;}
	inline void _putchar(char c){
		if(oued==oust+MAXBF)_flush(),oued=oubuf;
		*oued++=c;
		#ifdef local
		_flush();
		#endif
	}
	public:
	fastout(FILE*_ouf=stdout){
		oubuf=new char[MAXBF],ouf=_ouf,oust=oubuf,oued=oubuf;
	}
	~fastout(){_flush();delete[]oubuf;}
	template<typename Int> fastout&operator<<(Int n){
		if(n<0)_putchar('-'),n=-n;
		static char S[20];
		int t=0;
		do{S[t++]='0'+n%10,n/=10;}while(n);
		for(int i=0;i<t;++i)_putchar(S[t-i-1]);
		return *this;
	}
	fastout&operator<<(char c){_putchar(c);return *this;}
	fastout&operator<<(char*s){
		for(int i=0;s[i];++i)_putchar(s[i]);
		return *this;
	}
	fastout&operator<<(const char*s){
		for(int i=0;s[i];++i)_putchar(s[i]);
		return *this;
	}
}fo;
#define cin fi
#define cout fo
#else
#define czc ios::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#endif
#define mod7 1000000007
#define mod9 998244353
#define ll long long
#define isinf 0x3f3f3f3f
#define ilinf 0x7fffffff
#define lsinf 0x3f3f3f3f3f3f3f3f
#define llinf 0x7fffffffffffffff
#define pii pair<int,int>
#define fr first
#define sc second
#define next ___
#define pb push_back
#define N 35
#define M 5000010
#define For(i,a,b) for(ll i=(a);i<=(b);++i)
#define Rep(i,a,b,c) for(ll i=(a);i<=(b);i+=c)
#define Per(i,a,b,c) for(ll i=(a);i>=(b);i-=c)
#define Gra(i) for(ll i=h[x];i;i=next[i])
typedef int arr[N];
typedef int arm[M];
typedef ll arl[N];
typedef ll alm[M];
typedef double ard[N];
typedef double adm[M];
namespace modint{
	template<typename Int,Int mod,Int m1>
	struct modint{
		Int v;
		modint(){v=0;}
		modint(Int a){v=a;}
		bool operator==(modint a){return v==a.v;}
		bool operator!=(modint a){return v!=a.v;}
		bool operator<(modint a){return v<a.v;}
		bool operator>(modint a){return v>a.v;}
		bool operator<=(modint a){return v<=a.v;}
		bool operator>=(modint a){return v>=a.v;}
		bool operator==(Int a){return v==a;}
		bool operator!=(Int a){return v!=a;}
		bool operator<(Int a){return v<a;}
		bool operator>(Int a){return v>a;}
		bool operator<=(Int a){return v<=a;}
		bool operator>=(Int a){return v>=a;}
		friend bool operator==(Int a,modint b){return b==a;}
		friend bool operator!=(Int a,modint b){return b!=a;}
		friend bool operator<(Int a,modint b){return b>a;}
		friend bool operator>(Int a,modint b){return b<a;}
		friend bool operator<=(Int a,modint b){return b>=a;}
		friend bool operator>=(Int a,modint b){return b<=a;}
		modint operator+(modint a){return v>=mod-a.v?v-mod+a.v:v+a.v;}
		modint operator-(modint a){return v>=a.v?v-a.v:v+mod-a.v;}
		modint modnum(modint a){
			return a-((__int128(a.v)*m1)>>80)*mod;
		}
		modint operator*(modint a){return modnum(v*a.v);}
		modint operator+=(modint a){*this=*this+a;return *this;}
		modint operator-=(modint a){*this=*this-a;return *this;}
		modint operator*=(modint a){*this=*this*a;return *this;}
		modint qpow(modint a,Int b){
			modint r(1);
			for(;b;b>>=1,a*=a)if(b&1)r*=a;
			return r;
		}
		modint operator/(modint a){return qpow(a,mod-2)*v;}
		modint operator/=(modint a){*this=*this/a;return *this;}
		modint&operator++(){*this=*this+1;return *this;}
		modint&operator--(){*this=*this-1;return *this;}
		const modint operator++(int){modint r=*this;++*this;return r;}
		const modint operator--(int){modint r=*this;--*this;return r;}
		friend modint operator+(Int a,modint b){return b+a;}
		friend modint operator-(Int a,modint b){return b-a;}
		friend modint operator*(Int a,modint b){return b*a;}
		friend modint operator/(Int a,modint b){return modint(a)/b;}
		#ifdef cout
		friend fastin&operator>>(fastin&in,modint&n){in>>n.v;return in;}
		friend fastout&operator<<(fastout&out,modint n){out<<n.v;return out;}
		#else
		friend istream&operator>>(istream&in,modint&n){in>>n.v;return in;}
		friend ostream&operator<<(ostream&out,modint n){out<<n.v;return out;}
		#endif
	};
	typedef modint<long long,1000000007,1208925811152148> int7;
	typedef modint<long long,998244353,1211051999424262> int9;
}
using namespace modint;
int n,m,s,t,k,id[N],a[N],f[1<<10|1][N][N],d[N][N],vis[N][N],mn[N][N];
vector<int>e[N];
vector<pii>p[N][N];
void re(int s,int x,int y){
	f[s][x][y]=-mn[x][y];
	vis[x][y]=1;
	for(auto [i,j]:p[x][y]){
		mn[i][j]=min(mn[i][j],f[s][x][y]);
		if(!--d[i][j]&&!vis[i][j]){
			re(s,i,j);
		}
	}
}
void solve(){
	cin>>n>>m>>s>>t;
	For(i,1,m){
		int x,y;
		cin>>x>>y;
		e[x].pb(y);
	};
	cin>>k;
	fill(id+1,id+1+n,k+1);
	For(i,1,k){
		int v,w;
		cin>>v>>w;
		a[i]=w;
		id[v]=i;
	};
	For(s,0,(1<<k)-1){
		For(i,1,n){
			For(j,1,n){
				vis[i][j]=d[i][j]=0;
				p[i][j].clear();
			};
		};
		For(i,1,n)if(!((s>>(id[i]-1))&1)){
			For(j,1,n)if(!((s>>(id[j]-1))&1)){
				mn[i][j]=isinf;
				for(int x:e[i]){
					if((s>>(id[x]-1))&1){
						mn[i][j]=min(mn[i][j],f[s^(1<<(id[x]-1))][j][x]-a[id[x]]);
					}else{
						p[j][x].pb({i,j});
						++d[i][j];
					}
				}
			};
		};
		For(i,1,n){
			For(j,1,n){
				if(!d[i][j]&&!vis[i][j]){
					re(s,i,j);
				}
			};
		};
		for(;;){
			int x=0,y=0,tem=0;
			For(i,1,n){
				For(j,1,n){
					if(!vis[i][j]&&mn[i][j]<tem){
						tem=mn[i][j];
						x=i;y=j;
					}
				};
			};
			if(!tem)break;
			re(s,x,y);
		}
		For(i,1,n){
			For(j,1,n){
				if(d[i][j]>0){
					f[s][i][j]=0;
				}
			};
		};
	};
	cout<<f[(1<<k)-1][s][t];
	return;
}
int main(){
	#ifndef zcz
	czc;
	#endif
	int t=1;
	while(t--)solve();
	return 0;
}