#include <iostream>
#include <vector>
#include <map>
#include <utility>
#include <climits>

using namespace std;

const double DOUBLE_MAX = 1000000000.0;

int
main()
{
  int n, m, p, a, b;
  while (cin >> n >> m >> p >> a >> b){
    if (n == 0 && m == 0 && p == 0 && a == 0 && b == 0){
      break;
    }
    int ticket[10];
    for (int i = 0; i < n; ++i){
      cin >> ticket[i];
    }
    vector<vector<pair<int, double> > > graph(m + 1);
    for (int i = 0; i < p; ++i){
      int from, to;
      double cost;
      cin >> from >> to >> cost;
      graph[from].push_back(make_pair(to, cost));
      graph[to].push_back(make_pair(from, cost));
    }
    vector<double> init_vec((1 << n), DOUBLE_MAX);
    vector<vector<double> > cache(m + 1, init_vec);
    multimap<double, pair<int, int> > map_open;
    map_open.insert(make_pair(0.0, make_pair(a, 0)));
    cache[a][0] = 0.0;
    while (!map_open.empty()){
      double t = map_open.begin()->first;
      pair<int, int> now = map_open.begin()->second;
      map_open.erase(map_open.begin());
      if (cache[now.first][now.second] != t){
        continue;
      }
      vector<pair<int, double> >& branch = graph[now.first];
      /*
      for (vector<pair<int, double> >::iterator it = branch.begin(); it != branch.end(); ++it){
        if (cache[it->first][now.second] > t + it->second){
          cache[it->first][now.second] = t + it->second;
          map_open.insert(make_pair(t + it->second, make_pair(it->first, now.second)));
        }
      */
      for (int i = 0; i < n; ++i){
        int now_ticket = now.second;
        if (now_ticket & (1 << i)){
          continue;
        }
        int next_ticket = now.second | (1 << i);
        for (vector<pair<int, double> >::iterator it = branch.begin(); it != branch.end(); ++it){
          double temp_cost = t + (it->second / (double)ticket[i]);
          //cout << now.first << " " << next_ticket << " " << t << endl;;
          if (cache[it->first][next_ticket] > temp_cost){
            cache[it->first][next_ticket] = temp_cost;
            //cout << it->first << " " << next_ticket << " " << temp_cost << endl;
            map_open.insert(make_pair(temp_cost, make_pair(it->first, next_ticket)));
          }
        }

      }

    }
    double answer = DOUBLE_MAX;
    /*
    for(int t = 0; t < (1 << n); ++t){
      for (int city = 1; city <= m; ++city){
        printf("%.02lf ", cache[city][t]);
      }
      printf("\n");
    }
    */
    for (int i = 0; i < (1 << n); ++i){
      //cout << answer << " ";
      if (answer > cache[b][i]){
        answer = cache[b][i];
      }
    }
    //cout << endl;
    if (answer >= DOUBLE_MAX){
      cout << "Impossible" << endl;
    } else {
      printf("%.05lf\n", answer);
    }
    //cout << endl;
  }
  return 0;
}