#include<bits/stdc++.h>
namespace my_std{
    using namespace std;
    #define pii pair<int,int>
    #define fir first
    #define sec second
    #define MP make_pair
    #define rep(i,x,y) for (int i=(x);i<=(y);i++)
    #define drep(i,x,y) for (int i=(x);i>=(y);i--)
    #define go(x) for (int i=head[x];i;i=edge[i].nxt)
    #define templ template<typename T>
	#define sz 33
    typedef long long ll;
    typedef double db;
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    templ inline T rnd(T l,T r) {return uniform_int_distribution<T>(l,r)(rng);}
    templ inline bool chkmax(T &x,T y){return x<y?x=y,1:0;}
    templ inline bool chkmin(T &x,T y){return x>y?x=y,1:0;}
    templ inline void read(T& t) {
        t=0;char f=0,ch=getchar();double d=0.1;
        while(ch>'9'||ch<'0') f|=(ch=='-'),ch=getchar();
        while(ch<='9'&&ch>='0') t=t*10+ch-48,ch=getchar();
        if(ch=='.'){ch=getchar();while(ch<='9'&&ch>='0') t+=d*(ch^48),d*=0.1,ch=getchar();}
        t=(f?-t:t);
    }
    template<typename T,typename... Args>inline void read(T& t,Args&... args){read(t); read(args...);}
    void file() {
        #ifdef zqj
        freopen("a.in","r",stdin);
        #endif
    }
    inline void chktime() {
        #ifdef zqj
        cerr<<clock()/1000.0<<'\n';
        #endif
    }
    #ifdef mod
    ll ksm(ll x,int y){ll ret=1;for (;y;y>>=1,x=x*x%mod) if (y&1) ret=ret*x%mod;return ret;}
    ll inv(ll x){return ksm(x,mod-2);}
    #else
    ll ksm(ll x,int y){ll ret=1;for (;y;y>>=1,x=x*x) if (y&1) ret=ret*x;return ret;}
    #endif
}
using namespace my_std;

int N;
ll f[sz*sz]; int type[sz*sz];
unordered_set<int>V1[sz*sz];
int deg[sz*sz];

priority_queue<pair<ll,int>>S;

int del[sz*sz];
void delete_vertex(int x);
void delete_edge(int x,int v) {
	assert(V1[v].count(x));
	V1[v].erase(x); --deg[x];
	if (chkmax(f[x],-f[v])) S.push({f[x],x});
	if (!del[x]&&!deg[x]) delete_vertex(x);
}
void delete_vertex(int x) {
	del[x]=1;
	while (V1[x].size()) delete_edge(*(V1[x].begin()),x);
};

void solve() {
	while (S.size()) S.pop();
	rep(i,0,N-1) S.push({f[i],i});
	while (S.size()) {
		auto [w,x]=S.top(); S.pop();
		if (del[x]||f[x]!=w) continue;
		if (w<=0) {
			rep(x,0,N-1) if (!del[x]) f[x]=0;
			return;
		}
		delete_vertex(x);
	}
}

int n,m,A,B,K;
int e[sz][sz];
int pos[sz],id[sz]; ll w[sz];
ll dp[1<<10][sz][sz];

int main() {
    file();
    read(n,m,A,B),--A,--B;
	int x,y;
	while (m--) read(x,y),e[x-1][y-1]=1;
	read(K);
	rep(i,0,n-1) id[i]=-1;
	rep(i,0,K-1) read(pos[i],w[i]),--pos[i],id[pos[i]]=i;
	rep(s,0,(1<<K)-1) {
		N=n*n;
		memset(f,~0x3f,sizeof(f));
		rep(i,0,n-1) if (id[i]==-1||!(s>>id[i]&1)) rep(j,0,n-1) if (id[j]==-1||!(s>>id[j]&1)) {
			int x=i*n+j;
			rep(k,0,n-1) if (e[i][k]) {
				if (id[k]==-1||!(s>>id[k]&1)) {
					int y=j*n+k;
					++deg[x],V1[y].insert(x);
				}
				else {
					ll ww=w[id[k]];
					ww-=dp[1<<id[k]^s][j][k];
					chkmax(f[x],ww);
				}
			}
		}
		solve();
		rep(i,0,n-1) if (id[i]==-1||!(s>>id[i]&1)) rep(j,0,n-1) if (id[j]==-1||!(s>>id[j]&1)) {
			int x=i*n+j;
			dp[s][i][j]=f[x];
			V1[x]=unordered_set<int>(),deg[x]=del[x]=0;
		}
	}
	cout<<dp[(1<<K)-1][A][B]<<'\n';
	return 0;
}