#include<iostream>

using namespace std;

void out(int last ,int n){
	for(int i=1;i<last;i++)
		cout << i << endl;
	for(int i=n-1;i>=last;i--)
		cout << i << endl;
}


int main(){
	int n,l,prev;
	cin >> n >> l >> prev;
	for(int i = 1;i<n;i++){
		int a;
		cin >> a;
		if(prev + a >= l){
			cout << "Possible" << endl;
			out(i,n);
			return 0;
		}
		prev = a;
	}
	cout << "Impossible" << endl;
	return 0;
}