#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <sstream>
#include <iomanip>
using namespace std;
typedef long long LL;
const double INF=0x4fffffff;
const double EXP=1e-5;
const int MS=10;
const int SIZE=31;

double dp[1<<MS][SIZE];

int edges[SIZE][SIZE];
int cnt[MS];

int start,final,tickets,citys,edge;

void solve()
{
      for(int i=0;i<(1<<tickets);i++)
            fill(dp[i],dp[i]+SIZE,INF);
      dp[(1<<tickets)-1][start-1]=0;
      double res=INF;
      for(int i=(1<<tickets)-1;i>=0;i--)
      {
            res=min(res,dp[i][final-1]);
            for(int u=0;u<citys;u++)
            {
                  for(int j=0;j<tickets;j++)
                  {
                        if((i>>j)&1)
                        {
                              for(int v=0;v<citys;v++)
                                    if(edges[u][v]>=0)
                                    dp[i-(1<<j)][v]=min(dp[i-(1<<j)][v],dp[i][u]+double(edges[u][v])/cnt[j]);
                        }
                  }
            }
      }
      if(res+EXP>=INF)
            printf("Impossible\n");
      else
            printf("%.3lf\n",res);
}

int main()
{
      while(scanf("%d%d%d%d%d",&tickets,&citys,&edge,&start,&final)==5&&tickets)
      {

            for(int i=0;i<tickets;i++)
            {
                  scanf("%d",&cnt[i]);
            }
            memset(edges,-1,sizeof(edges));
            for(int i=0;i<edge;i++)
            {
                  int x,y,z;
                  scanf("%d%d%d",&x,&y,&z);
                  edges[x-1][y-1]=edges[y-1][x-1]=z;
            }
            solve();
      }
      return 0;
}