#include <cstdio>
#define INFTY (1<<20) // Infinity
#define UNDET (1<<21) // Undetermined

int n, m, p, a, b;
int t[8];
int adj[32][32];

int mask[8] = {0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80};
int left_tickets[9] = {0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};

//              city  tickets
double min_time[ 32 ][  256  ];
// min_time[i][j] contains time to reach city-b when the traveler started from city-i with <j>-ticket(s).
// <j>-ticket(s) means... if j & mask[k] == mask[k], then the traveler has ticket-k.

double tsp(int cur, int left_ticket, int visited[32], double wasted_time)
{
  double local_min = INFTY;
  if (cur == b) { return wasted_time; }
  if (left_ticket == 0) { return INFTY; }
  if (min_time[cur][left_ticket] != UNDET) { return wasted_time + min_time[cur][left_ticket]; }

  for (int i = 0; i < m; i++) {
    if (adj[cur][i] < INFTY && !visited[i]) {
      for (int use_tkt = 0; use_tkt < n; use_tkt++) {
        if (!(left_ticket & mask[use_tkt])) { continue; }

        double result;
        visited[i] = 1;
        result = tsp(i,
                     (left_ticket &~ mask[use_tkt]),
                     visited,
                     wasted_time + ((double)adj[cur][i]) / (t[use_tkt]));
        if (result < local_min) { local_min = result; }
        visited[i] = 0;
      }
    }
  }

  if (local_min < INFTY) { min_time[cur][left_ticket] = local_min - wasted_time; }
  else                   { min_time[cur][left_ticket] = INFTY; }
  return local_min;
}

int main()
{
  int i, j, visited[32];

  while (1) {
    scanf("%d%d%d%d%d", &n, &m, &p, &a, &b); if (n == 0) { break; } a--; b--;
    for (i = 0; i < m; i++) { for (j = 0; j < m; j++) { adj[i][j] = INFTY; } }
    for (i = 0; i < n; i++) { scanf("%d", &t[i]); }
    for (i = 0; i < p; i++) {
      int x, y, z;
      scanf("%d%d%d", &x, &y, &z); x--; y--;
      adj[x][y] = adj[y][x] = z;
    }

    for (i = 0; i < m; i++) { visited[i] = 0; }
    for (i = 0; i < m; i++) { for (j = 0; j <= left_tickets[n]; j++) { min_time[i][j] = UNDET; } }
    visited[a] = 1;

    double result = tsp(a, left_tickets[n], visited, 0.0);
    if (result < INFTY) { printf("%.3lf\n", result); }
    else { printf("Impossible\n"); }
  }
  return 0;
}