using System; using System.Collections.Generic; using System.Linq; using System.IO; using System.Threading; using System.Text; using System.Text.RegularExpressions; using System.Diagnostics; using static util; using P = pair; using Number = System.Int64; class Program { static void Main(string[] args) { var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false }; var solver = new Solver(sw); // var t = new Thread(solver.solve, 1 << 28); // 256 MB // t.Start(); // t.Join(); solver.solve(); sw.Flush(); } } class Solver { StreamWriter sw; Scan sc; void Prt(string a) => sw.WriteLine(a); void Prt(IEnumerable a) => Prt(string.Join(" ", a)); void Prt(params object[] a) => Prt(string.Join(" ", a)); public Solver(StreamWriter sw) { this.sw = sw; this.sc = new Scan(); } public void solve() { int n, s, t, k; sc.Multi(out n, out s, out t, out k); --s; --t; int N = n * k; var edge = new List>[N]; var rev = new List>[N]; for (int i = 0; i < N; i++) { edge[i] = new List>(); rev[i] = new List>(); } var x = sc.LongArr; int m = sc.Int; for (int i = 0; i < m; i++) { int u, v; long c; sc.Multi(out u, out v, out c); --u; --v; c += x[v]; for (int j = 0; j < k - 1; j++) { edge[u * k + j].Add(new pair(c, v * k + j + 1)); rev[v * k + j + 1].Add(new pair(c, u * k + j)); } edge[u * k + k - 1].Add(new pair(c, v * k + k - 1)); rev[v * k + k - 1].Add(new pair(c, u * k + k - 1)); } var dist = ShortestPath.Dijkstra(edge, s * k, LM); if (dist[t * k + k - 1] == LM) { Prt("Impossible"); return; } var lis = new List(); int now = t * k + k - 1; while (now != s * k) { lis.Add(now / k + 1); foreach (var item in rev[now]) { if (dist[item.v2] == dist[now] - item.v1) { now = item.v2; break; } } } lis.Add(s + 1); lis.Reverse(); Prt("Possible"); Prt(dist[t * k + k - 1] + x[s]); Prt(lis.Count); Prt(lis); } } class pair : IComparable> { public T v1; public U v2; public pair() : this(default(T), default(U)) {} public pair(T v1, U v2) { this.v1 = v1; this.v2 = v2; } public int CompareTo(pair a) { int c = Comparer.Default.Compare(v1, a.v1); return c != 0 ? c : Comparer.Default.Compare(v2, a.v2); } public override string ToString() => $"{v1} {v2}"; public void Deconstruct(out T a, out U b) { a = v1; b = v2; } } static class util { public static readonly int M = 1000000007; // public static readonly int M = 998244353; public static readonly long LM = 1L << 60; public static readonly double eps = 1e-11; public static void DBG(string a) => Console.Error.WriteLine(a); public static void DBG(IEnumerable a) => DBG(string.Join(" ", a)); public static void DBG(params object[] a) => DBG(string.Join(" ", a)); public static void Assert(params bool[] conds) { if (conds.Any(x => !x)) throw new Exception(); } public static pair make_pair(T v1, U v2) => new pair(v1, v2); public static int CompareList(IList a, IList b) where T : IComparable { for (int i = 0; i < a.Count && i < b.Count; i++) if (a[i].CompareTo(b[i]) != 0) return a[i].CompareTo(b[i]); return a.Count.CompareTo(b.Count); } public static bool inside(int i, int j, int h, int w) => i >= 0 && i < h && j >= 0 && j < w; public static readonly int[] dd = { 0, 1, 0, -1 }; // static readonly string dstring = "RDLU"; public static IEnumerable

adjacents(int i, int j) { for (int k = 0; k < 4; k++) yield return new P(i + dd[k], j + dd[k ^ 1]); } public static IEnumerable

adjacents(int i, int j, int h, int w) { for (int k = 0; k < 4; k++) if (inside(i + dd[k], j + dd[k ^ 1], h, w)) yield return new P(i + dd[k], j + dd[k ^ 1]); } public static IEnumerable

adjacents(this P p) => adjacents(p.v1, p.v2); public static IEnumerable

adjacents(this P p, int h, int w) => adjacents(p.v1, p.v2, h, w); public static IEnumerable all_subset(this int p) { for (int i = 0; ; i = i - p & p) { yield return i; if (i == p) break; } } public static Dictionary compress(this IEnumerable a) => a.Distinct().OrderBy(v => v).Select((v, i) => new { v, i }).ToDictionary(p => p.v, p => p.i); public static Dictionary compress(params IEnumerable[] a) => compress(a.SelectMany(x => x)); public static T[] inv(this Dictionary dic) { var res = new T[dic.Count]; foreach (var item in dic) res[item.Value] = item.Key; return res; } public static void swap(ref T a, ref T b) where T : struct { var t = a; a = b; b = t; } public static void swap(this IList a, int i, int j) where T : struct { var t = a[i]; a[i] = a[j]; a[j] = t; } public static T[] copy(this IList a) { var ret = new T[a.Count]; for (int i = 0; i < a.Count; i++) ret[i] = a[i]; return ret; } } class Scan { StreamReader sr; public Scan() { sr = new StreamReader(Console.OpenStandardInput()); } public Scan(string path) { sr = new StreamReader(path); } public int Int => int.Parse(Str); public long Long => long.Parse(Str); public double Double => double.Parse(Str); public string Str => ReadLine.Trim(); public string ReadLine => sr.ReadLine(); public pair Pair() { T a; U b; Multi(out a, out b); return new pair(a, b); } public P P => Pair(); public int[] IntArr => StrArr.Select(int.Parse).ToArray(); public long[] LongArr => StrArr.Select(long.Parse).ToArray(); public double[] DoubleArr => StrArr.Select(double.Parse).ToArray(); public string[] StrArr => Str.Split(new[]{' '}, StringSplitOptions.RemoveEmptyEntries); bool eq() => typeof(T).Equals(typeof(U)); T ct(U a) => (T)Convert.ChangeType(a, typeof(T)); T cv(string s) => eq() ? ct(int.Parse(s)) : eq() ? ct(long.Parse(s)) : eq() ? ct(double.Parse(s)) : eq() ? ct(s[0]) : ct(s); public void Multi(out T a) => a = cv(Str); public void Multi(out T a, out U b) { var ar = StrArr; a = cv(ar[0]); b = cv(ar[1]); } public void Multi(out T a, out U b, out V c) { var ar = StrArr; a = cv(ar[0]); b = cv(ar[1]); c = cv(ar[2]); } public void Multi(out T a, out U b, out V c, out W d) { var ar = StrArr; a = cv(ar[0]); b = cv(ar[1]); c = cv(ar[2]); d = cv(ar[3]); } public void Multi(out T a, out U b, out V c, out W d, out X e) { var ar = StrArr; a = cv(ar[0]); b = cv(ar[1]); c = cv(ar[2]); d = cv(ar[3]); e = cv(ar[4]); } public void Multi(out T a, out U b, out V c, out W d, out X e, out Y f) { var ar = StrArr; a = cv(ar[0]); b = cv(ar[1]); c = cv(ar[2]); d = cv(ar[3]); e = cv(ar[4]); f = cv(ar[5]); } } static class ShortestPath { public static void WarshallFloyd(Number[][] g) { int n = g.Length; for (int i = 0; i < n; ++i) for (int j = 0; j < n; ++j) for (int k = 0; k < n; ++k) g[j][k] = Math.Min(g[j][k], g[j][i] + g[i][k]); } class Heap { int n; Number[] values; int[] keys, indices; void Update(int i, Number val) { if (indices[i] == -1) { keys[n] = i; indices[i] = n; ++n; } values[i] = val; int p = indices[i]; while (p > 0) { int par = p - 1 >> 1; if (values[keys[par]] > val) { keys[p] = keys[par]; indices[keys[p]] = p; p = par; } else break; } keys[p] = i; indices[i] = p; } int Pop() { --n; int ret = keys[0]; indices[ret] = -1; int i = keys[n]; keys[n] = -1; Number val = values[i]; if (n == 0) return ret; int p = 0; while ((p << 1 | 1) < n) { int ch = p << 1 | 1; if (ch < n - 1 && values[keys[ch]] > values[keys[ch + 1]]) ++ch; if (val > values[keys[ch]]) { keys[p] = keys[ch]; indices[keys[p]] = p; p = ch; } else break; } keys[p] = i; indices[i] = p; return ret; } void init(int m, int s, Number inf) { n = 1; values = new Number[m]; keys = new int[m]; indices = new int[m]; for (int i = 0; i < m; i++) { values[i] = inf; keys[i] = -1; indices[i] = -1; } values[s] = 0; keys[0] = s; indices[s] = 0; } void init(Number[] dist) { n = dist.Length; values = dist; keys = new int[n]; for (int i = 0; i < n; i++) keys[i] = i; Array.Sort(keys, (x, y) => values[x].CompareTo(values[y])); indices = new int[n]; for (int i = 0; i < n; i++) indices[keys[i]] = i; } public void Run(List>[] edges, Number[] dist) { init(dist); while (n > 0) { int p = Pop(); foreach (var e in edges[p]) if (values[e.v2] > values[p] + e.v1) Update(e.v2, values[p] + e.v1); } } public Number[] Run(List>[] edges, int s, Number inf) { init(edges.Length, s, inf); while (n > 0) { int p = Pop(); foreach (var e in edges[p]) if (values[e.v2] > values[p] + e.v1) Update(e.v2, values[p] + e.v1); } return values; } public pair Run(List>[] edges, int s, long Mod, Number inf) { init(edges.Length, s, inf); var cnts = new long[edges.Length]; cnts[s] = 1; while (n > 0) { int p = Pop(); foreach (var e in edges[p]) if (values[e.v2] > values[p] + e.v1) { cnts[e.v2] = cnts[p]; Update(e.v2, values[p] + e.v1); } else if (values[e.v2] == values[p] + e.v1) cnts[e.v2] = (cnts[e.v2] + cnts[p]) % Mod; } return new pair(values, cnts); } } public static Number[] Dijkstra(List>[] edges, int s, Number inf) => new Heap().Run(edges, s, inf); public static void Dijkstra(List>[] edges, Number[] dist) => new Heap().Run(edges, dist); public static pair Dijkstra(List>[] edges, int s, long Mod, Number inf) => new Heap().Run(edges, s, Mod, inf); }