import java.io.PrintWriter

fun main(args : Array<String>) {
//    for (i in 1..100000) print("1 ")
//    if (true) return
    val printWriter = PrintWriter(System.out)
    fun println(s : String) = printWriter.println(s)
    fun println(i : Int) = printWriter.println(i)
    val (n, l) = readLine()!!.split(' ').map(String::toInt)
    val a = readLine()!!.split(' ').map(String::toInt)
    var q = -1
    for (i in 0..n - 2) {
        if (a[i] + a[i + 1] >= l) q = i;
    }
    if (q == -1) println("Impossible") else {
        println("Possible")
        for (i in 0..q - 1) println(i + 1)
        for (i in n - 2 downTo q) println(i + 1)
    }
    printWriter.close()
}