#include <stdio.h>
#include <stdlib.h>
#define MN 100000
int n,l,a[MN+5],b;
int main(){
	scanf("%d%d",&n,&l);
	for(int i=1;i<=n;i++)
		scanf("%d",&a[i]);
	for(b=1;b<=n-1;b++)
		if(a[b]+a[b+1]>=l)break;
	if(b==n) puts("Impossible"),exit(0);
	puts("Possible");
	for(int i=1;i<b;i++)
		printf("%d\n",i);
	for(int i=n-1;i>b;i--)
		printf("%d\n",i);
	printf("%d\n",b);
}