#include <bits/stdc++.h>

#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

typedef long long LL;
template<typename T> using V = std::vector<T>;
template<typename T> using VV = std::vector<std::vector<T>>;
template<typename T> using VVV = std::vector<std::vector<std::vector<T>>>;

template<class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
template<class T> inline void print(T x) { std::cout << x << std::endl; }
template<class T> inline void print_vec(const std::vector<T> &v) { for (int i = 0; i < v.size(); ++i) {  if (i != 0) {std::cout << " ";} std::cout << v[i];} std::cout << "\n"; }
template<class T> inline bool inside(T y, T x, T H, T W) {return 0 <= y and y < H and 0 <= x and x < W; }
template<class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) { return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)); }
template<class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }

const int INF = 1L << 30;
const double EPS = 1e-9;
const std::string YES = "YES", Yes = "Yes", NO = "NO", No = "No";
const std::vector<int> dy4 = { 0, 1, 0, -1 }, dx4 = { 1, 0, -1, 0 };    // 4近傍（右, 下, 左, 上）
const std::vector<int> dy8 = { 0, -1, 0, 1, 1, -1, -1, 1 }, dx8 = { 1, 0, -1, 0, 1, 1, -1, -1 };

using namespace std;


int main() {
    cin.tie(0);
	ios::sync_with_stdio(false);

	while (true) {
	    int N, M, P, A, B;
	    cin >> N >> M >> P >> A >> B;
	    A--; B--;
	    if (N == 0 and M == 0) {
	        break;
	    }

	    vector<int> T(N);
	    FOR(i, 0, N) {
	        cin >> T[i];
	    }
	    vector<vector<pair<int, int>>> graph(M);
	    FOR(i, 0, P) {
	        int x, y, z;
	        cin >> x >> y >> z;
	        x--; y--;
	        graph[x].emplace_back(make_pair(y, z));
	        graph[y].emplace_back(make_pair(x, z));
	    }

	    vector<vector<double>> dp(M, vector<double>(1 << N, INT_MAX));
	    dp[A][0] = 0;

	    queue<pair<int, int>> que;
	    que.push(make_pair(A, 0));
	    while (not que.empty()) {
	        int now = que.front().first;
	        int b = que.front().second;
	        que.pop();

	        FOE(p, graph[now]) {
	            int next = p.first;
	            double dist = p.second;

                FOR(i, 0, N) {
                    if (b & 1 << i) {
                        continue;
                    }

                    double new_dist = dp[now][b] + dist / T[i];
                    if (dp[next][b | (1 << i)] > new_dist) {
                    	dp[next][b | (1 << i)] = new_dist;
						que.push(make_pair(next, b | (1 << i)));
                    }
                }
	        }
	    }

		double ans = INF;
	    FOR(b, 0, dp[0].size()) {
	    	ans = min(ans, dp[B][b]);
	    }
	    if (ans == INF) {
	    	print("Impossible");
	    }
	    else {
	    	cout << fixed << setprecision(5) << ans << endl;
	    }

	}

	return 0;
}


Measured memory_bytes: 3332