Problem:
Problem D: Traveling by Stagecoach

Once upon a time, there was a traveler.

He plans to travel using stagecoaches (horse wagons).
His starting point and destination are fixed,
but he cannot determine his route.
Your job in this problem is to write a program
which determines the route for him.

There are several cities in the country,
and a road network connecting them.
If there is a road between two cities, one can travel by a stagecoach
from one of them to the other.
A coach ticket is needed for a coach ride.
The number of horses is specified in each of the tickets.
Of course, with more horses, the coach runs faster.

At the starting point, the traveler has a number of coach tickets.
By considering these tickets and the information on the road network,
you should find the best possible route that takes him to the destination
in the shortest time.
The usage of coach tickets should be taken into account.

The following conditions are assumed.

A coach ride takes the traveler from one city to another
directly connected by a road.
In other words, on each arrival to a city, he must change the coach.

Only one ticket can be used for a coach ride between two cities
directly connected by a road.

Each ticket can be used only once.

The time needed for a coach ride is the distance between two cities
divided by the number of horses.

The time needed for the coach change should be ignored.

Input

The input consists of multiple datasets, each in the following format.
The last dataset is followed by a line containing five zeros
(separated by a space).

n m p a b

t1 t2 ... tn

x1 y1 z1

x2 y2 z2

...

xp yp zp

Every input item in a dataset is a non-negative integer.
If a line contains two or more input items, they are separated by a space.

n is the number of coach tickets.
You can assume that the number of tickets is between 1 and 8.
m is the number of cities in the network.
You can assume that the number of cities is between 2 and 30.
p is the number of roads between cities, which may be zero.

a is the city index of the starting city.
b is the city index of the destination city.
a is not equal to b.
You can assume that all city indices in a dataset (including the above two)
are between 1 and m.

The second line of a dataset gives the details of coach tickets.
ti is the number of horses specified in the i-th
coach ticket (1<=i<=n).
You can assume that the number of horses is between 1 and 10.

The following p lines give the details of roads between cities.
The i-th road connects two cities with city indices
xi and yi,
and has a distance zi (1<=i<=p).
You can assume that the distance is between 1 and 100.

No two roads connect the same pair of cities.
A road never connects a city with itself.
Each road can be traveled in both directions.

Output

For each dataset in the input, one line should be output as specified below.
An output line should not contain extra characters such as spaces.

If the traveler can reach the destination, the time needed
for the best route (a route with the shortest time) should be printed.
The answer should not have an error greater than 0.001.
You may output any number of digits after the decimal point,
provided that the above accuracy condition is satisfied.

If the traveler cannot reach the destination,
the string "Impossible" should be printed.
One cannot reach the destination either when there are no routes
leading to the destination, or when the number of tickets is not sufficient.
Note that the first letter of "Impossible" is in uppercase,
while the other letters are in lowercase.

Sample Input

3 4 3 1 4
3 1 2
1 2 10
2 3 30
3 4 20
2 4 4 2 1
3 1
2 3 3
1 3 3
4 1 2
4 2 5
2 4 3 4 1
5 5
1 2 10
2 3 10
3 4 10
1 2 0 1 2
1
8 5 10 1 5
2 7 1 8 4 5 6 3
1 2 5
2 3 4
3 4 7
4 5 3
1 3 25
2 4 23
3 5 22
1 4 45
2 5 51
1 5 99
0 0 0 0 0

Output for the Sample Input

30.000
3.667
Impossible
Impossible
2.856

Solution in JAVA (judge verdict: Accepted)

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.InputMismatchException;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;

public class Main {

	static PrintWriter out;
	static InputReader ir;
	static boolean debug = false;

	static void solve() {
		for (;;) {
			int n = ir.nextInt();
			int m = ir.nextInt();
			int p = ir.nextInt();
			int a = ir.nextInt() - 1;
			int b = ir.nextInt() - 1;
			if (n == 0 && m == 0 && p == 0)
				return;
			int[] t = ir.nextIntArray(n);
			Graph[] g = new Graph[m];
			for (int i = 0; i < m; i++)
				g[i] = new Graph();
			for (int i = 0; i < p; i++) {
				int x = ir.nextInt() - 1;
				int y = ir.nextInt() - 1;
				int z = ir.nextInt();
				g[x].add(new int[] { y, z });
				g[y].add(new int[] { x, z });
			}
			double[][] d = new double[m][1 << n];
			for (int i = 0; i < m; i++)
				Arrays.fill(d[i], 1 << 30);
			PriorityQueue<double[]> pq = new PriorityQueue<double[]>(new Comparator<double[]>() {
				public int compare(double[] A, double[] B) {
					return Double.compare(A[2], B[2]);
				}
			});
			d[a][(1 << n) - 1] = 0;
			pq.add(new double[] { a, (1 << n) - 1, 0 });
			while (!pq.isEmpty()) {
				double[] from = pq.poll();
				for (int i = 0; i < g[(int) from[0]].size(); i++) {
					int[] to = g[(int) from[0]].get(i);
					for (int j = 0; j < n; j++) {
						if (((int) from[1] & (1 << j)) != 0
								&& d[to[0]][(int) from[1] ^ (1 << j)] > d[(int) from[0]][(int) from[1]]
										+ (double) to[1] / t[j]) {
							d[to[0]][(int) from[1] ^ (1 << j)] = d[(int) from[0]][(int) from[1]]
									+ (double) to[1] / t[j];
							pq.add(new double[] { to[0], (int) from[1] ^ (1 << j),
									d[(int) to[0]][(int) from[1] ^ (1 << j)] });
						}
					}
				}
			}
			double mi = 1 << 30;
			for (int i = 0; i < 1 << n; i++)
				mi = Math.min(mi, d[b][i]);
			if (mi == 1 << 30) {
				out.println("Impossible");
			} else {
				out.println(mi);
			}
		}
	}

	static class Graph extends ArrayList<int[]> {
	}

	public static void main(String[] args) throws Exception {
		ir = new InputReader(System.in);
		out = new PrintWriter(System.out);
		solve();
		out.flush();
	}

	static class InputReader {

		private InputStream in;
		private byte[] buffer = new byte[1024];
		private int curbuf;
		private int lenbuf;

		public InputReader(InputStream in) {
			this.in = in;
			this.curbuf = this.lenbuf = 0;
		}

		public boolean hasNextByte() {
			if (curbuf >= lenbuf) {
				curbuf = 0;
				try {
					lenbuf = in.read(buffer);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (lenbuf <= 0)
					return false;
			}
			return true;
		}

		private int readByte() {
			if (hasNextByte())
				return buffer[curbuf++];
			else
				return -1;
		}

		private boolean isSpaceChar(int c) {
			return !(c >= 33 && c <= 126);
		}

		private void skip() {
			while (hasNextByte() && isSpaceChar(buffer[curbuf]))
				curbuf++;
		}

		public boolean hasNext() {
			skip();
			return hasNextByte();
		}

		public String next() {
			if (!hasNext())
				throw new NoSuchElementException();
			StringBuilder sb = new StringBuilder();
			int b = readByte();
			while (!isSpaceChar(b)) {
				sb.appendCodePoint(b);
				b = readByte();
			}
			return sb.toString();
		}

		public int nextInt() {
			if (!hasNext())
				throw new NoSuchElementException();
			int c = readByte();
			while (isSpaceChar(c))
				c = readByte();
			boolean minus = false;
			if (c == '-') {
				minus = true;
				c = readByte();
			}
			int res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res = res * 10 + c - '0';
				c = readByte();
			} while (!isSpaceChar(c));
			return (minus) ? -res : res;
		}

		public long nextLong() {
			if (!hasNext())
				throw new NoSuchElementException();
			int c = readByte();
			while (isSpaceChar(c))
				c = readByte();
			boolean minus = false;
			if (c == '-') {
				minus = true;
				c = readByte();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res = res * 10 + c - '0';
				c = readByte();
			} while (!isSpaceChar(c));
			return (minus) ? -res : res;
		}

		public double nextDouble() {
			return Double.parseDouble(next());
		}

		public int[] nextIntArray(int n) {
			int[] a = new int[n];
			for (int i = 0; i < n; i++)
				a[i] = nextInt();
			return a;
		}

		public long[] nextLongArray(int n) {
			long[] a = new long[n];
			for (int i = 0; i < n; i++)
				a[i] = nextLong();
			return a;
		}

		public char[][] nextCharMap(int n, int m) {
			char[][] map = new char[n][m];
			for (int i = 0; i < n; i++)
				map[i] = next().toCharArray();
			return map;
		}
	}

	static void tr(Object... o) {
		if (debug)
			out.println(Arrays.deepToString(o));
	}
}