#include "bits/stdc++.h"
#include <unordered_set>
#define rep(i,n) for(ll i = 0; i < n; i++)
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define vll vector<vector<long long>>
#define vl vector<long long>
#define vi vector<int>
#define vii vector<vector<int>>
#define pb push_back
#define pf push_front
#define ld long double
#define Sort(a) sort(a.begin(),a.end())
#define cSort(a,cmp) sort(a.begin(),a.end(),cmp)
#define reSort(a) sort(a.rbegin(), a.rend())
static const ll llMAX = numeric_limits<long long>::max();
static const int intMAX = numeric_limits<int>::max();
static const ll  llMIN = numeric_limits<long long>::min();
static const int intMIN = numeric_limits<int>::min();
static const ll d_5 = 100000;
static const ll d9_7 = 1000000007;
static const ll d_9 = 1000000000;
static const double PI=3.14159265358979323846;
template<class T>
void Printvector(std::vector<T> a){
  int size = a.size();
  rep(i,size){
    cout<<a[i]<<" ";
  }
  cout<<endl;
}
template<class T>
void Printvector(std::vector<std::vector<T>> a){
  int size = a.size();
  rep(i,size){
    int size2=a[i].size();
    rep(j,size2){
      cout<<a[i][j]<<" ";
    }
    cout<<endl;
  }
  cout<<endl;
}
ll digitpower(ll a,ll b){//aのb乗を計算
  if(b==1){
    return a;
  }else if(b==0){
    return 1;
  }
  int mode=0;
  if(b%2==1){
    ll tmp = digitpower(a,(b-1)/2);
    if(mode==1){
      tmp%=d9_7;
    }
    tmp*=tmp;
    if(mode==1){
      tmp%=d9_7;
    }
    tmp*=a;
    if(mode==1){
      return tmp%d9_7;
    }else{
      return tmp;
    }
  }else{
    ll tmp = digitpower(a,(b)/2);
    if(mode==1){
      tmp%=d9_7;
    }
    tmp*=tmp;
    if(mode==1){
      tmp%=d9_7;
    }
    if(mode==1){
      return tmp%d9_7;
    }else{
      return tmp;
    }
  }
}
vl facs(2000010,-1);
ll Factrial(ll num){
  if(facs[num]!=-1){
    return facs[num];
  }
  if(num==1||num<=0){
    return 1;
  }else if(num<0){
    printf("ERROR_minus\n");
    return 0;
  }else{
    facs[num]=(num*Factrial(num-1))%d9_7;
    return facs[num];
  }
}
long long modinv(long long a, long long m) {//modの逆元

    long long b = m, u = 1, v = 0;
    while (b) {
        long long t = a / b;
        a -= t * b; swap(a, b);
        u -= t * v; swap(u, v);
    }
    u %= m; 
    if (u < 0) u += m;
    return u;
}

vl invs(2000010,-1);
ll linercomb(ll n,ll k, ll mod){//n,kの線形時間で求める
if(n<k)return 0;
if(n<0)return 0;
if(k==0 || k==n)return 1;
  ll ans=Factrial(n);
  if(invs[k]==-1){
    invs[k]=modinv(Factrial(k),mod);
  }
  ans*=invs[k];
  ans%=d9_7;
  ll k1=Factrial(n-k);
  k1%=mod;
  ans*=modinv(k1,mod);
  ans%=mod;
  return ans;
}
unordered_map<ll,ll> prime_factor(int64_t n) {
  unordered_map<ll,ll> ret;
  for(int64_t i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}

template<class T>
vector<T> getaccum(vector<T> a){
  int size=a.size();
  vector<T> ans(size);
  ans[0]=a[0];
  for(int i=0;i<size-1;i++){
    ans[i+1]=ans[i]+a[i+1];
    //ans[i+1]%=d9_7;
  }
  return ans;
}
struct datas{
  ld w;
  ld p;
};/*
bool cmp(const datas &a, const datas &b)
{
    return a.num < b.num;
}*/
ll getaccumnum(vector<ll> accum,int l,int r){//閉区間[l,r]の総和
  if(l==0){
    return accum[r];
  }else{
    return accum[r]-accum[l-1];
  }
}
int main(void){
  ll l;
  int n;
  cin>>n>>l;
  vl a(n);
  rep(i,n)cin>>a[i];
  deque<ll> nums;
  ll nowsum=0;
  rep(i,n){
    nums.push_back(a[i]);
    nowsum+=a[i];
  }
  int flag=0;
  int index;
  rep(i,n-1){
    if(a[i]+a[i+1]>=l){
      flag=1;
      index=i;
      break;
    }
  }
  if(flag==0){
    cout<<"Impossible"<<endl;
    return 0;
  }
  int nowl=0;
  int nowr=n-1;
  int nowlen=n;
  vi ans;
  int ltime=max(0,index);
  int rtime=max(0,n-1-(index+1));
      rep(i,ltime){

      ans.pb(nowl+1);
      nowsum-=nums.front();
      nowlen--;
      nowl++;
      nums.pop_front();
      }
  rep(i,rtime){

      ans.pb(nowr);
      nowsum-=nums.back();
      nowlen--;
      nowr--;
      nums.pop_back();
  }
    
  ans.pb(index+1);
    cout<<"Possible"<<endl;
  rep(i,n-1){
    cout<<ans[i]<<endl;
  }
}

//<<std::setprecision(30)

Measured memory_bytes: 34188