#include <bits/stdc++.h>
#include <unistd.h>

using namespace std;

#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}


typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx[4] = { -1,0, 1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
//#define mp make_pair


int main() {
  while (true) {
    int n, m, p, a, b;
    cin >> n >> m >> p >> a >> b;
    if (n == 0 && m == 0) return 0;
    a--; b--;
    vi t(n);
    rep (i, n) {
      cin >> t[i];
    }
    vector<vector<pair<double, int> > > G(m);
    rep (i, p) {
      int u, v;
      double d;
      cin >> u >> v >> d;
      u--; v--;
      G[u].push_back({d, v});
      G[v].push_back({d, u});
    }

    int s = 1 << n;
    vector<vector<double> > dist(m, vector<double>(s, inf));
    dist[a][s - 1] = 0;
    priority_queue<pair<double, pii>, vector<pair<double, pii> >, greater<pair<double, pii> > > pq;
    pq.push({0, pii(a, s - 1)});
    while (!pq.empty()) {
      int u = pq.top().second.first, state = pq.top().second.second;
      double cost = pq.top().first;
      pq.pop();
      rep (i, n) {
        int mask = 1 << i;
        if (!(mask & state)) continue;
        int next_state = state - mask;
        rep (j, G[u].size()) {
          int v = G[u][j].second;
          double next_cost = cost + G[u][j].first / t[i];
          if (dist[v][next_state] > next_cost) {
            dist[v][next_state] = next_cost;
            pq.push({next_cost, pii(v, next_state)});
          }
        }
      }
    }
    
    double ans = inf;
    rep (i, s) {
      chmin(ans, dist[b][i]);
    }
    if (ans >= inf / 2) {
      cout << "Impossible" << endl;
    }
    else {
      Sp(ans);
    }
  }
}



Measured memory_bytes: 4584