#include<cstdio>
#include<cstring>
using namespace std;
#define MAXN 100000
long long n,l;
long long fl=0;
long long a[MAXN+5];
long long read()
{
	long long x=0,f=1;
	char c=getchar();
	while('0'>c||c>'9'){if(c=='-') f=-1;c=getchar();}
	while('0'<=c&&c<='9'){x=x*10+c-'0';c=getchar();}
	return x*f;
}
int main()
{
	n=read(),l=read();
	for(int i=1;i<=n;i++)
	a[i]=read();
	for(int i=1;i<n;i++)
	{
		if(a[i]+a[i+1]>=l)
		{
			fl=i;
			break;
		}
	}
	if(!fl)
	{
		printf("Impossible\n");
	}
	else 
	{
		printf("Possible\n");
		for(int i=1;i<=fl-1;i--)
			printf("%d\n",i);
		for(int i=n-1;i>=fl;i--)
			printf("%d\n",i);
	}
}
