//test
#include<bits/stdc++.h>
using namespace std;
namespace my_std{
	#define ll int
	#define bl bool
	ll my_pow(ll a,ll b,ll mod){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res=(res*a)%mod;
			a=(a*a)%mod;
			b>>=1;
		}
		return res;
	}
	ll qpow(ll a,ll b){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res*=a;
			a*=a;
			b>>=1;
		}
		return res;
	}
	#define db double
	#define pf printf
	#define pc putchar
	#define fr(i,x,y) for(register ll i=(x);i<=(y);i++)
	#define pfr(i,x,y) for(register ll i=(x);i>=(y);i--)
	#define go(u) for(ll i=head[u];i;i=e[i].nxt)
	#define enter pc('\n')
	#define space pc(' ')
	#define fir first
	#define sec second
	#define MP make_pair
	#define il inline
	#define inf 2e9
	#define random(x) rand()*rand()%(x)
	#define inv(a,mod) my_pow((a),(mod-2),(mod))
	il ll read(){
		ll sum=0,f=1;
		char ch=0;
		while(!isdigit(ch)){
			if(ch=='-') f=-1;
			ch=getchar();
		}
		while(isdigit(ch)){
			sum=sum*10+(ch^48);
			ch=getchar();
		}
		return sum*f;
	}
	il void write(ll x){
		if(x<0){
			x=-x;
			pc('-');
		}
		if(x>9) write(x/10);
		pc(x%10+'0');
	}
	il void writeln(ll x){
		write(x);
		enter;
	}
	il void writesp(ll x){
		write(x);
		space;
	}
}
using namespace my_std;
vector<ll> in[33],out[33];
ll n,m,k,A,B,pos[11],W[11],mp[33],f[1100][33][33][2],lst[33][33][2],g[33][33][2];
void solve(ll S){
	fr(i,1,n) fr(j,1,n) fr(l,0,1) f[S][i][j][l]=g[i][j][l]=-inf;
	fr(i,1,k){
		if(!(S&(1ll<<(i-1)))) continue;
		fr(j,0,(ll)in[pos[i]].size()-1){
			ll u=in[pos[i]][j];
			fr(v,1,n) f[S][u][v][0]=max(f[S][u][v][0],W[i]-f[S^(1ll<<(i-1))][pos[i]][v][1]);
			fr(v,1,n) f[S][v][u][1]=max(f[S][v][u][1],W[i]-f[S^(1ll<<(i-1))][v][pos[i]][0]);
		}
	}
	fr(_,1,n*n){
		fr(u,1,n) fr(v,1,n) fr(o,0,1) lst[u][v][o]=f[S][u][v][o];
		fr(u,1,n){
			fr(v,1,n){
				fr(i,0,(ll)out[u].size()-1){
					ll w=out[u][i];
					if(f[S][w][v][1]!=-inf) f[S][u][v][0]=max(f[S][u][v][0],-f[S][w][v][1]);
				}
				fr(i,0,(ll)out[v].size()-1){
					ll w=out[v][i];
					if(f[S][u][w][0]!=-inf) f[S][u][v][1]=max(f[S][u][v][1],-f[S][u][w][0]);
				}
			}
		}
		bl ck=0;
		fr(u,1,n) fr(v,1,n) fr(o,0,1) if(lst[u][v][o]!=f[S][u][v][o]) ck=1;
		if(!ck) break;
	}
	fr(u,1,n) fr(v,1,n) fr(o,0,1) if(f[S][u][v][o]==-inf) f[S][u][v][o]=0;
}
int main(){
	n=read();
	m=read();
	A=read();
	B=read();
	fr(i,1,m){
		ll u=read(),v=read();
		out[u].push_back(v);
		in[v].push_back(u);
	}
	k=read();
	fr(i,1,k){
		pos[i]=read();
		W[i]=read();
		mp[pos[i]]=i;
	}
	fr(i,1,(1ll<<k)-1) solve(i);
	write(f[(1ll<<k)-1][A][B][0]);
}