#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <queue>
#include <map>
#include <algorithm>
#include <set>
using namespace std;
#define MM(a) memset(a,0,sizeof(a))
typedef long long ll;
typedef unsigned long long ULL;
const int mod = 1000000007;
const double eps = 1e-10;
const int inf = 0x3f3f3f3f;
const int big=50000;
int max(int a,int b) {return a>b?a:b;};
double min_(double a,double b) {return a<b?a:b;};
int n,m,p,a,b,t[10];
struct e{
   int to;double cost;
};
vector<e> G[35];
double dp[1<<10][35];

void solve()
{
    double ans=inf;
    for(int s=0;s<=(1<<n)-1;s++)
      fill(dp[s]+1,dp[s]+m+1,inf);//不能用memset初始化，因为其只能对整型
    dp[(1<<n)-1][a]=0;
    for(int s=(1<<n)-1;s>=0;s--)
    {
        ans=min_(ans,dp[s][b]);
        for(int u=1;u<=m;u++)//枚举起点u
          for(int j=1;j<=n;j++)//使用第j张票
            if(s&(1<<(j-1)))
               for(int i=0;i<G[u].size();i++)
                  {
                      int v=G[u][i].to;
                      double c=G[u][i].cost;
                     dp[s&(~(1<<(j-1)))][v]=min_(dp[s&(~(1<<(j-1)))][v],dp[s][u]+c/t[j]);//状态转移方程
                  }
    }
    if(ans>=inf-1)
        printf("Impossible\n");
    else
        printf("%.3f\n",ans);
}

int main()
{
    while(~scanf("%d %d %d %d %d",&n,&m,&p,&a,&b))
    {
       if(!n&&!m&&!p&&!a&&!b) break;
       for(int i=1;i<=n;i++)
               scanf("%d",&t[i]);
       for(int i=1;i<=m;i++)
               G[i].clear();

       int f,t;double c;
       for(int i=1;i<=p;i++)
        {
            scanf("%d %d %lf",&f,&t,&c);
            G[f].push_back((e){t,c});
            G[t].push_back((e){f,c});
        }

       solve();
    }
    return 0;
}