#include "bits/stdc++.h"
using namespace std;
int main() {
	long long N, L, bA, A, P;
	bool check = false;
	cin >> N >> L >> bA;
	for (int i = 1; i < N; i++) {
		cin >> A;
		if (bA + A >= L) P = i, check = true;
		bA = A;
	}
	if (check) {
		cout << "Possible" << endl;
		for (int i = 1; i < P; i++) cout << i << endl;
		for (int i = N - 1; i >= P; i--) cout << i << endl;
	}
	else cout << "Impossible" << endl;
}