#include <stdio.h>

	int N,L;
	int a[100000];
	int i,j,ans;


int main(void){
	scanf("%d %d",&N,&L);

	for(i=0;i<N;i++){
		scanf("%d",&a[i]);
	}
	ans = N;
	for(i=0;i<N-1;i++){
		if(a[i]+a[i+1]>=L){
			ans = i;
		}
	}

	if(ans == N){
		printf("Impossible\n");
	}else{
		printf("Possible\n");
		for(i=0;i<ans;i++){
			printf("%d\n",i+1);
		}
		for(i=N-2;i>=ans;i--){
			printf("%d\n",i+1);
		}
	}
	return 0;

}