import java.io.*;
import java.util.*;

class Main {

    public static InputReader in;
    public static PrintWriter out;
    
    public static final int MOD = (int) (1e9 + 7);
    public static void main(String[] args) {
        in = new InputReader(System.in);
        out = new PrintWriter(System.out);
        
        int N = in.nextInt(),
              L = in.nextInt();
        PriorityQueue<Node> pq = new PriorityQueue<Node>();
        int[] a = new int[N];
        int maxValue = Integer.MIN_VALUE;
        int index = -1;
        for (int i = 0; i < N; i++) {
            a[i] = in.nextInt();
            pq.add(new Node(a[i], i+1));
            if(i != 0 && a[i] + a[i-1] > maxValue) {
                maxValue = a[i] + a[i-1];
                index = i;
            }
        }
        if(maxValue < L) {
            out.println("Impossible");
            out.close();
            return;
        }
        
        StringBuilder result = new StringBuilder();
        for (int i = 1; i < index; i++)     result.append(i + "\n");
        for (int i = N-1; i >= index; i--)  result.append(i + "\n");
        out.println("Possible");
        out.print(result.toString());
        out.close();
    }
    
    static class Node implements Comparable<Node>{
        int value, index;
        
        public Node (int value, int index) {
            this.value = value;
            this.index = index;
        }
        
        public void print() {
            out.println(value + " " + index + " ");
        }
        
        public int compareTo(Node that) {
            return Integer.compare(this.value, that.value);
        }
    }
    
    static class InputReader {

        private InputStream stream;
        private byte[] buf = new byte[8192];
        private int curChar, snumChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int snext() {
            if (snumChars == -1)
                throw new InputMismatchException();
            if (curChar >= snumChars) {
                curChar = 0;
                try {
                    snumChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (snumChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = snext();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nextLong() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = snext();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

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

        public String readString() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = snext();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }
    }
}

Measured memory_bytes: 20052