#include <bits/stdc++.h>
using namespace std;
#define fo(i,a,b) for (int i = (a); i < (b); i++)
#define N 1234567

int n, l, a[N];
vector<int> ans;
int main() {
	scanf("%d %d", &n, &l);
	fo(i,0,n) scanf("%d", a+i);
	fo(i,0,n-1) if (a[i] + a[i+1] >= l) {
		puts("Possible");
		fo(j,0,i) printf("%d\n", j+1);
		for (int j = n-2; j >= i; j--) printf("%d\n", j+1);
		return 0;
	} 
	puts("Impossible");


	return 0;
}