#include <cstdio>
#include <algorithm>
using namespace std;

#define N 310
#define inf (INT_MAX/2)
int n,m,s,t,g[N][N],dp[N][N],fmax,qmax,cs[N];

int main() {
  scanf("%d%d",&n,&m);
	scanf("%d%d%d%d",&s,&t,&fmax,&qmax);
	--s,--t;
	for (int i=0;i<n;++i) scanf("%d",cs+i);

	fill(&g[0][0],&g[n][0],inf);
	for (int i=0;i<n;++i) g[i][i]=0;
	for (int i=0;i<m;++i) {
		int a,b,c; scanf("%d%d%d",&a,&b,&c); --a,--b;
		g[a][b]=g[b][a]=c;
	}
	for (int i=0;i<n;++i)
		for (int j=0;j<n;++j)
			for (int k=0;k<n;++k)
				g[j][k]<?=g[j][i]+g[i][k];

	fill(&dp[0][0],&dp[qmax+1][0],-inf);
	for (int i=0;i<n;++i) 
		if (g[s][i]<=fmax) 
			dp[1][i]=(fmax-g[s][i])*cs[i];

	for (int q=1;q<qmax;++q) 
		for (int i=0;i<n;++i) 
			if (dp[q][i]!=-inf)
				for (int j=0;j<n;++j) 
					if (g[i][j]<=fmax) 
						dp[q+1][j]>?=dp[q][i]+max(fmax*(cs[j]-cs[i])-cs[j]*g[i][j],-cs[i]*g[i][j]);

	int ans=-inf;
	for (int i=0;i<n;++i) 
		if (g[i][t]<=fmax&&dp[qmax][i]!=-inf) 
			ans>?=dp[qmax][i]-g[i][t]*cs[i];
	if (ans<-inf/2) puts("impossible");
	else printf("%d\n",ans);
}