#include <cstdio>

 #include <cstdlib>

 #include <cstring>

 #include <climits>

 #include <cctype>

 #include <cmath>

 #include <string>

 #include <sstream>

 #include <iostream>

 #include <algorithm>

 #include <iomanip>

 using namespace std;

 #include <queue>

 #include <stack>

 #include <vector>

 #include <deque>

 #include <set>

 #include <map>

 typedef long long LL;

 typedef long double LD;

 #define INFF 0x3f3f3f3f

 #define INF 2139062143

 #define pi acos(-1.0)

 #define lson l, m, rt<<1

 #define rson m+1, r, rt<<1|1

 typedef pair<int, int> PI;

 typedef pair<int, PI> PP;

 #ifdef _WIN32

 #define LLD "%I64d"

 #else

 #define LLD "%lld"

 #endif

 //#pragma comment(linker, "/STACK:1024000000,1024000000")

 //LL quick(LL a, LL b){LL ans=1;while(b){if(b & 1)ans*=a;a=a*a;b>>=1;}return ans;}

 //inline int read(){char ch=' ';int ans=0;while(ch<'0' || ch>'9')ch=getchar();while(ch<='9' && ch>='0'){ans=ans*10+ch-'0';ch=getchar();}return ans;}

 //inline void print(LL x){printf(LLD, x);puts("");}

 //inline void read(int &x){char c = getchar();while(c < '0') c = getchar();x = c - '0'; c = getchar();while(c >= '0'){x = x * 10 + (c - '0'); c = getchar();}}

 int t[];

 int mp[][];

 double dp[<<][];  // 剩下的车票状态  现在在v的最小花费

 int main()

 {

 #ifndef ONLINE_JUDGE

     freopen("in.txt", "r", stdin);

     freopen("out.txt", "w", stdout);

 #endif

     int n, m, p, a, b;

     while(~scanf("%d%d%d%d%d", &n, &m, &p, &a, &b) && (n || m || p || a || b))

     {

         for(int i=;i<n;i++)

             scanf("%d", &t[i]);

         memset(mp, -, sizeof(mp));

         while(p--)

         {

             int u, v, w;

             scanf("%d%d%d", &u, &v, &w);

             u--, v--;

             if(mp[u][v]<)

                 mp[u][v]=mp[v][u]=w;

             else

                 mp[u][v]=mp[v][u]=min(mp[u][v], w);

         }

         memset(dp, , sizeof(dp));

         dp[(<<n)-][a-]=;

         double ans=INF;

         for(int s=(<<n)-;s>=;s--)

         {

             ans=min(ans, dp[s][b-]);

             for(int v=;v<m;v++)

                 for(int i=;i<n;i++)

                     if(s>>i & )

                         for(int u=;u<m;u++)

                             if(mp[v][u]>=)

                                 dp[s & ~(<<i)][u]=min(dp[s & ~(<<i)][u], dp[s][v]+mp[v][u]*1.0/t[i]);

    // 使用车票i , v->u

         }

         if(ans==INF)

             printf("Impossible\n");

         else

             printf("%.3lf\n", ans);

     }

     return ;

 }