using System.Collections.Generic;
using static System.Math;
using System;

public class PriorityQueue<T> where T : IComparable
{
    private IComparer<T> _comparer = null;
    private int _type = 0;
    private T[] _heap;
    private int _sz = 0;
    private int _count = 0;
    /// <summary>
    /// Priority Queue with custom comparer
    /// </summary>
    public PriorityQueue(int maxSize, IComparer<T> comparer)
    {
        _heap = new T[maxSize];
        _comparer = comparer;
    }

    /// <summary>
    /// Priority queue
    /// </summary>
    /// <param name="maxSize">max size</param>
    /// <param name="type">0: asc, 1:desc</param>
    public PriorityQueue(int maxSize, int type = 0)
    {
        _heap = new T[maxSize];
        _type = type;
    }
    private int Compare(T x, T y)
    {
        if (_comparer != null) return _comparer.Compare(x, y);
        return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
    }
    public void Push(T x)
    {
        _count++;
        //node number
        var i = _sz++;
        while (i > 0)
        {
            //parent node number
            var p = (i - 1) / 2;

            if (Compare(_heap[p], x) <= 0) break;

            _heap[i] = _heap[p];
            i = p;
        }
        _heap[i] = x;
    }
    public T Pop()
    {
        _count--;
        T ret = _heap[0];
        T x = _heap[--_sz];

        int i = 0;
        while (i * 2 + 1 < _sz)
        {
            //children
            int a = i * 2 + 1;
            int b = i * 2 + 2;

            if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

            if (Compare(_heap[a], x) >= 0) break;

            _heap[i] = _heap[a];
            i = a;
        }
        _heap[i] = x;
        return ret;
    }
    public int Count()
    {
        return _count;
    }
    public T Peek()
    {
        return _heap[0];
    }
    public bool Contains(T x)
    {
        for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
        return false;
    }
    public void Clear()
    {
        while (this.Count() > 0) this.Pop();
    }
    public IEnumerator<T> GetEnumerator()
    {
        var ret = new List<T>();
        while (this.Count() > 0)
        {
            ret.Add(this.Pop());
        }
        foreach (var r in ret)
        {
            this.Push(r);
            yield return r;
        }
    }
    public T[] ToArray()
    {
        T[] array = new T[_sz];
        int i = 0;
        foreach (var r in this)
        {
            array[i++] = r;
        }
        return array;
    }
}

public class Edge
{
    public int to1 { get; set; }
    public int to2 { get; set; }
    public double d { get; set; }
}
public class Node : IComparable
{
    public int id1 { get; set; }
    public int id2 { get; set; }
    public double d { get; set; }
    public int CompareTo(object obj)
    {
        var x = (Node)obj;
        if (this.d > x.d) return 1;
        else if (this.d == x.d) return 0;
        else return -1;
    }
}

public class hello
{
    public static void Main()
    {
        while (true)
        {
            string[] line = Console.ReadLine().Trim().Split(' ');
            var n = int.Parse(line[0]);
            var m = int.Parse(line[1]);
            var p = int.Parse(line[2]);
            var a = int.Parse(line[3]) - 1;
            var b = int.Parse(line[4]) - 1;
            if (n == 0 && m == 0 && p == 0 && a == -1 && b == -1) break;
            var jmax = 1 << n;
            line = Console.ReadLine().Trim().Split(' ');
            var t = Array.ConvertAll(line, double.Parse);
            var aa = getEdge(n, m, p, t, jmax);
            var mind = new double[m, jmax];
            for (int i = 0; i < m; i++)
                for (int j = 0; j < jmax; j++) mind[i, j] = 9999999999999d;
            goDijk(aa, a, 0, m, jmax, mind);
            print(mind, b, jmax);
        }
    }
    static void print(double[,] mind, int b, int jmax)
    {
        var ans = 9999999999999d;
        for (int i = 0; i < jmax; i++)
            if (mind[b, i] != 9999999999999d) ans = Min(ans, mind[b, i]);
        if (ans == 9999999999999d) Console.WriteLine("Impossible");
        else Console.WriteLine(ans);
    }
    static List<Edge>[,] getEdge(int n, int m, int p, double[] t, int jmax)
    {
        var aa = new List<Edge>[m, jmax];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < jmax; j++) aa[i, j] = new List<Edge>();
        for (int i = 0; i < p; i++)
        {
            string[] line = Console.ReadLine().Trim().Split(' ');
            var x = int.Parse(line[0]) - 1;
            var y = int.Parse(line[1]) - 1;
            var z = double.Parse(line[2]);
            for (int j = 0; j < jmax; j++)
                for (int k = 0; k < n; k++)
                    if (((j >> k) & 1) == 0)
                    {
                        var ww2 = (j | (1 << k));
                        var d = z / t[n - k - 1];
                        aa[x, j].Add(new Edge { d = d, to1 = y, to2 = ww2 });
                        aa[y, j].Add(new Edge { d = d, to1 = x, to2 = ww2 });
                    }
        }
        return aa;
    }

    static void goDijk(List<Edge>[,] aa, int r1, int r2, int m, int jmax, double[,] mind)
    {
        var pq = new PriorityQueue<Node>(jmax + 100000);
        pq.Push(new Node { id1 = r1, id2 = r2, d = 0d });
        while (pq.Count() > 0)
        {
            var targ = pq.Pop();
            var nowd = targ.d;
            var nowid1 = targ.id1;
            var nowid2 = targ.id2;
            if (mind[nowid1, nowid2] < nowd) continue;
            mind[nowid1, nowid2] = nowd;
            foreach (var x in aa[nowid1, nowid2])
                if (nowd + x.d < mind[x.to1, x.to2])
                {
                    mind[x.to1, x.to2] = nowd + x.d;
                    pq.Push(new Node { id1 = x.to1, id2 = x.to2, d = nowd + x.d });
                }
        }
    }
}



Measured memory_bytes: 133488