// 2005�‘“à—\‘ID Traveling by Stagecoach

#include <iostream>
#include <vector>
#include <queue>
#include <cstring>
#include <stdio.h>

using namespace std;

class Node{
public:
	int pos, ticket; double cost;
	Node(int pos, int ticket, double cost) : pos(pos), ticket(ticket), cost(cost) {}
	bool operator < (const Node &nd) const { return cost > nd.cost; }
};

int main(){
	int n, m, p, a, b;
	int t[8];
	bool visit[256][31];
	double dist[256][31];
	while(cin >> n >> m >> p >> a >> b, n){
		for(int i=0;i<n;i++) cin >> t[i];
		vector< vector< pair<int, double> > > g(m+1);
		for(int i=0;i<p;i++){
			int x, y, z; cin >> x >> y >> z;
			g[x].push_back(make_pair(y,z));
			g[y].push_back(make_pair(x,z));
		}
		for(int i=0;i<(1<<n);i++)
			for(int j=1;j<=m;j++) dist[i][j] = 1e12;
		dist[(1<<n)-1][a] = 0;
		memset(visit, false, sizeof(visit));
		priority_queue<Node> qu; qu.push(Node(a, (1<<n)-1, 0));
		double res = -1;
		while(!qu.empty()){
			Node nd = qu.top(); qu.pop();
			int pos = nd.pos, tic = nd.ticket; double ct = nd.cost;
			if(visit[tic][pos]) continue;
			visit[tic][pos] = true;
			if(pos == b){ res = ct; break; }
			for(int i=0;i<g[pos].size();i++){
				int next = g[pos][i].first;
				for(int j=0;j<n;j++){
					if(!(tic&(1<<j))) continue;
					if(dist[tic^(1<<j)][next] > dist[tic][pos] + g[pos][i].second/t[j]){
						dist[tic^(1<<j)][next] = dist[tic][pos] + g[pos][i].second/t[j];
						qu.push(Node(next, tic^(1<<j), dist[tic^(1<<j)][next]));
					}
				}
			}
		}
		if(res < 0) puts("Impossible");
		else        printf("%.6lf\n", res);
	}
}