#include<bits/stdc++.h>
#define rep(i,x,y) for (int i=(x);i<=(y);i++)

using namespace std;

const int N=1e5+100;
int n,m,a[N];

int main(){
	scanf("%d%d",&n,&m);
	rep (i,1,n) scanf("%d",&a[i]);
	int p=0;
	rep (i,2,n) if (a[i-1]+a[i]>=m){p=i-1; break;}
	if (!p) return puts("Impossible"),0;
	puts("Possible");
	rep (i,1,p-1) printf("%d\n",i);
	for (int i=n-1;i>p;i--) printf("%d\n",i);
	printf("%d\n",p);
	return 0;
}