import std.stdio;
import std.conv, std.array, std.algorithm, std.string;
import std.math, std.random, std.range, std.datetime;
import std.bigint;

void main(){
	long n, l;
	{
		long[] tmp = readln.chomp.split.map!(to!long).array;
		n = tmp[0], l = tmp[1];
		}
	
	long[] as = readln.chomp.split.map!(to!long).array;
	
	string ans;
	int[] ans2;
	
	long center = -1;
	for(int i = 0; i < as.length - 1; i ++){
		if(as[i] + as[i + 1] >= l){
			center = i;
			break;
			}
		}
	
	if(center == -1) ans = "Impossible";
	else{
		ans = "Possible";
		for(int i = 0; i < center; i ++){
			ans2 ~= i + 1;
			}
		for(int i = cast(int) (as.length - 2); i >= center; i --){
			ans2 ~= i + 1;
			}
		}
	
	ans.writeln;
	foreach(i; ans2) i.writeln;
	
	
	}