#include<bits/stdc++.h>
using namespace std;
typedef pair<int,int> P;
int main(){
  int n,l,a[100010],p=-1;
  cin>>n>>l;
  bool ans=0;
  priority_queue<P> q;
  for(int i=0;i<n;i++){
    cin>>a[i];
    if(i&&a[i]+a[i-1]>=l)ans=1,p=i;
  }
  if(ans){
    cout<<"Possible"<<endl;
    for(int i=1;i<p;i++)cout<<i<<endl;
    for(int i=n-1;i>p;i--)cout<<i<<endl;
    cout<<p<<endl;
  }
  else cout<<"Impossible"<<endl;
  return 0;
}
