#include <iostream>
using namespace std;

int N,L,arr[100001];

int main()
{
  cin>>N>>L;
  for (int i=0;i<N;i++)
    cin>>arr[i];
  int ans=-1;
  for (int i=0;i+1<N;i++)
    if (arr[i]+arr[i+1]>=L)
      ans=i+1;
  if (ans==-1){
    cout<<"Impossible"<<endl;
    return 0;
  }
  cout<<"Possible"<<endl;
  for (int i=1;i<ans;i++)
    cout<<i<<endl;
  for (int i=N-1;i>ans;i--)
    cout<<i<<endl;
  cout<<ans<<endl;
}
