#include<bits/stdc++.h>
using namespace std;

#define rep(i,l,r) for(int i=l;i<=r;++i)
#define per(i,r,l) for(int i=r;i>=l;--i)
const int N=1e5+5;
int a[N];

int main()
{
	//freopen("1.in","r",stdin);
	int n,l;
	cin>>n>>l;
	rep(i,1,n)scanf("%d",a+i);
	rep(i,1,n-1)
	if(a[i]+a[i+1]>=l)
	{
		puts("Possible");
		rep(j,1,i-1)printf("%d\n",j);
		per(j,n-1,i+1)printf("%d\n",j);
		printf("%d\n",i);
		exit(0);
	}
	puts("Impossible");
}