#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <functional>
#include <stdio.h>
#include <map>
using namespace std;
const int infinity = 1 << 29;
int table[40][40];
int m;
int n;
int connect[40][40];
int t[10];
double high;

double Solve(int src, int dest) {
  bool check[40];
  memset(check, 0, sizeof(check));
  high = 1e+30;
  multimap<double, pair<int, int> > up;
  up.insert(make_pair(0.0, make_pair(src, 0)));
  int count = 0;
  while(!up.empty()) {
    count++;
    double cost = up.begin()->first;
    int p = up.begin()->second.first;
    int d = up.begin()->second.second;
    up.erase(up.begin());
    //cout << p << ' ' << dest << ' ' << d << ' ' << cost << endl;
    if(p == dest){
      high = cost;
      //cout << "count = " << count << endl;
      return high;
    }
    if(d >= n){
      continue;
    }
    if(check[p])
      continue;
    check[p] = true;
    for(int i = 0; i < m; ++i) {
      if(table[p][i] == infinity)
	continue;
      up.insert(make_pair(cost+((double)table[p][i])/t[d], make_pair(i, d+1)));
    }
  }
  //cout << "Count = " << count << endl;
  return high;
}

int main(){
  while(true){
    int p, a, b;
    cin >> n >> m >> p >> a >> b;
    if(n == 0 && m == 0 and p == 0 and a == 0 and b == 0){
      break;
    }
    a--; b--;
    bool checked[m];
    for(int i = 0; i < m; i++){
      for(int j = 0; j < m; j++){
	table[i][j] = infinity;
      }
      checked[i] = false;
    }
    for(int i = 0; i < n; i++){
      cin >> t[i];
    }
    sort(t, t + n, greater<int>());
    for(int i = 0; i < p; i++){
      int x, y, z;
      cin >> x >> y >> z;
      x--; y--;
      table[x][y] = z;
      table[y][x] = z;
    }
    for(int i = 0; i < m; ++i)
      for(int j = 0; j < m; ++j) {
	connect[i][j] = (table[i][j] < infinity) ? 1 : infinity;
      }
    for(int k = 0; k < m; ++k)
      for(int i = 0; i < m; ++i)
	for(int j = 0; j < m; ++j) {
	  if(connect[i][j] > connect[i][k] + connect[k][j]){
	    connect[i][j] = connect[i][k] + connect[k][j];
	  }
	}
    if(connect[a][b] > n){
      cout << "Impossible" << endl;
      continue;
    }
    sort(t, t+n);
    double ans = 1e+30;
    int Count = 0;
    do {
      //double t = Solve(a, b);
      Solve(a, b);
      Count++;
      if(high < ans)
	ans = high;
    } while(next_permutation(t, t+n));
    if(ans > 1e+20){
      cout << "Impossible" << endl;
    }else{
      cout << ans << endl;
    }
  }
  return 0;
}