#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

template<typename D, Int toMaxQuery, Int useAssert = 1>
class RMQ{
public :
  struct T{ //遅延データ
    Int type; //0 - empty, 1 - update, 2 - ifUpdate
    D value;
    T():type(0){}
    T(Int type, D value):type(type), value(value){}
  };

  Int n, n_;
  D initD;        //datの初期値
  D returnD;      //範囲外の時に返す値
  vector<D> dat;
  vector<T> td;

  RMQ(){n=-1;}
  RMQ(Int n_, D initD):n_(n_), initD(initD), returnD(initD){
    n=1;
    while(n<n_)n *= 2;
    td.resize(2*n-1, T());
    dat.resize(2*n-1, initD);
  }

  //目的データのマージ
  inline D mergeD(D l, D r){return toMaxQuery? max(l, r):min(l, r);}

  //遅延データのマージ
  inline T mergeT(const T &from,const T &to){
    if(to.type == 0) return from;
    if(from.type == 0) return to; //from-empty
    if(to.type == 1) return to; //to-update;
    if(to.type == 2) return T(from.type, mergeD(from.value, to.value)); //to-ifupdate
    if(useAssert) assert(0);
    return T();
  }

  //目的データに遅延データを反映
  inline void apply(D &a, const T &b){
    if(b.type == 0) return;
    if(b.type == 1) a = b.value; //update
    else a = mergeD(a, b.value); // if-update
  }

  D dfs(Int a, Int b,const T x, Int k, Int l, Int r){
    if(r <= a || b <= l) return x.type == 0? returnD : dat[k];
    if(a <= l && r <= b){
      td[k] = mergeT(td[k], x);
      apply(dat[k], x);
      return dat[k];
    }
    Int kl = k * 2 + 1, kr = k * 2 + 2;
    { //遅延を子に反映
      td[kl] = mergeT(td[kl], td[k]);
      td[kr] = mergeT(td[kr], td[k]);
      apply(dat[kl], td[k]);
      apply(dat[kr], td[k]);
      td[k] = T();
    }
    D vl = dfs(a, b, x, kl, l, (l+r)/2);
    D vr = dfs(a, b, x, kr, (l+r)/2, r);
    return x.type == 0? mergeD(vl,vr) : (dat[k] = mergeD(vl, vr));
  }

  //[l,r)の値をxに変更　update(l,r,x)
  void update(Int l,Int r, D x){
    if(useAssert) assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    dfs(l, r,  T(1, x), 0, 0, n);
  }

  //[l,r)の値をもしもxの方が小さければ変更　ifupdate(l,r,x)
  //toMaxのときは[l,r)の値をもしもxの方が大きければ変更　ifupdate(l,r,x)
  void ifupdate(Int l,Int r, D x){
    if(useAssert) assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    dfs(l, r,  T(2, x), 0, 0, n);
  }

  //[l,r)の最小値を得る　get(l,r);
  D get(Int l,Int r){
    if(useAssert) assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    return dfs(l, r, T(0, initD), 0 , 0 ,n);
  }
};

template<typename D, Int useAssert = 1>
class RSUQ{
public :
  struct T{ //遅延データ
    Int type; //0 - empty, 1 - update
    D value;
    T():type(0){}
    T(Int type, D value):type(type), value(value){}
  };

  Int n, n_;
  vector<D> dat;
  vector<T> td;

  RSUQ(){n=-1;}
  RSUQ(Int n_):n_(n_){
    n=1;
    while(n<n_) n *= 2;
    td.resize(2*n-1, T());
    dat.resize(2*n-1, 0);
  }

  //目的データのマージ
  inline D mergeD(D l, D r){return l + r;}

  //遅延データのマージ
  inline T mergeT(const T &from, const T &to){
    if(to.type == 0) return from;
    return to;
  }

  //目的データに遅延データを反映
  inline void apply(D &a, const T &b, Int len){
    if(b.type == 0) return;
    a = b.value * len; //update
  }

  D dfs(Int a, Int b,const T x, Int k, Int l, Int r){
    if(r <= a || b <= l) return x.type == 0? 0 : dat[k];
    if(a <= l && r <= b){
      td[k] = mergeT(td[k], x);
      apply(dat[k], x, r - l);
      return dat[k];
    }
    Int kl = k * 2 + 1, kr = k * 2 + 2;
    { //遅延を子に反映
      td[kl] = mergeT(td[kl], td[k]);
      td[kr] = mergeT(td[kr], td[k]);
      apply(dat[kl], td[k], (r - l)/2);
      apply(dat[kr], td[k], (r - l)/2);
      td[k] = T();
    }
    D vl = dfs(a, b, x, kl, l, (l+r)/2);
    D vr = dfs(a, b, x, kr, (l+r)/2, r);
    return x.type == 0? mergeD(vl,vr) : (dat[k] = mergeD(vl, vr));
  }

  //[l,r)の値をxに変更　update(l,r,x)
  void update(Int l,Int r, D x){
    if(useAssert) assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    dfs(l, r,  T(1, x), 0, 0, n);
  }

  //[l,r)の総和を得る　get(l,r);
  D get(Int l,Int r){
    if(useAssert) assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    return dfs(l, r, T(), 0 , 0 ,n);
  }
};


Int check(vector<Int> A, Int L, vector<Int> ans){
  Int N = A.size();
  if((Int)ans.size() != N - 1) return 0;
  RMQ<P, 0> seg(N, P(INF, INF));

  seg.update(0, N, P(0, N-1));
  RSUQ<Int> seg2(N);
  for(Int i=0;i<N;i++) seg2.update(i, i + 1, A[i]);
  for(Int i:ans){
    i--;
    Int l, r; tie(l, r) = seg.get(i, i + 1);
    Int length = seg2.get(l, r + 1);
    if(length < L) return 0;
    seg.update(l, i + 1, P(l, i));
    seg.update(i + 1, r + 1, P(i + 1, r));
  }
  return 1;
}

vector<Int> solve(vector<Int> A, Int L){
  Int N = A.size();
  vector<Int> res;
  for(Int i=0;i<N-1;i++){
    Int len = A[i] + A[i + 1];
    if(len < L && i == N - 2) return {};
    if(len < L) res.push_back(i + 1);
    else{
      for(Int j=N-2;j>=i;j--) res.push_back(j + 1);
      return res;
    }
  }
  assert(0);
  return {};
}

signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int N, L;
  cin>>N>>L;
  vector<Int> A(N);
  cin>>A;

  auto ans = solve(A, L);
  if(ans.empty()) cout<<"Impossible"<<endl;
  else{
    cout<<"Possible"<<endl;
    assert(check(A, L, ans));
    for(Int i=0;i<N-1;i++) cout<<ans[i]<<endl;
  }


  return 0;
}


Measured memory_bytes: 19820