#include <iostream>
#include <iomanip>
#include <sstream>
#include <vector>
#include <string>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <functional>
#include <iterator>
#include <limits>
#include <numeric>
#include <utility>
#include <cmath>
#include <cassert>
#include <cstdio>

using namespace std;
using namespace placeholders;

using LL = long long;
using ULL = unsigned long long;
using VI = vector< int >;
using VVI = vector< vector< int > >;
using VS = vector< string >;
using SS = stringstream;
using PII = pair< int, int >;
using VPII = vector< pair< int, int > >;
template < typename T = int > using VT = vector< T >;
template < typename T = int > using VVT = vector< vector< T > >;
template < typename T = int > using LIM = numeric_limits< T >;

template < typename T > inline istream& operator>>( istream &s, vector< T > &v ){ for ( T &t : v ) { s >> t; } return s; }
template < typename T > inline ostream& operator<<( ostream &s, const vector< T > &v ){ for ( int i = 0; i < int( v.size() ); ++i ){ s << ( " " + !i ) << v[i]; } return s; }
template < typename T > inline T fromString( const string &s ) { T res; istringstream iss( s ); iss >> res; return res; }
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); }

#define REP2( i, n ) REP3( i, 0, n )
#define REP3( i, m, n ) for ( int i = ( int )( m ); i < ( int )( n ); ++i )
#define GET_REP( a, b, c, F, ... ) F
#define REP( ... ) GET_REP( __VA_ARGS__, REP3, REP2 )( __VA_ARGS__ )
#define FOR( e, c ) for ( auto &&e : c )
#define ALL( c ) begin( c ), end( c )
#define AALL( a, t ) ( t* )a, ( t* )a + sizeof( a ) / sizeof( t )
#define DRANGE( c, p ) ( c ).begin(), ( c ).begin() + ( p ), ( c ).end()

#define SZ( v ) ( (int)( v ).size() )
#define EXIST( c, e ) ( ( c ).find( e ) != ( c ).end() )

template < typename T > inline bool chmin( T &a, const T &b ){ if ( b < a ) { a = b; return true; } return false; }
template < typename T > inline bool chmax( T &a, const T &b ){ if ( a < b ) { a = b; return true; } return false; }

#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter

#define MP make_pair
#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl


class UnionFind
{
private:
	const int N;

	vector<int> parent;
	vector<int> rank;

	int groups_;
	std::vector< int > sizes;

public:
	UnionFind( int n ) : N( n ), parent( N ), rank( N, 0 ), groups_( N ), sizes( N, 1 )
	{
		std::iota( parent.begin(), parent.end(), 0 );

		return;
	}

	int find( int x )
	{
		if ( parent[x] == x )
		{
			return x;
		}
		return parent[x] = find( parent[x] );
	}

	bool same( int x, int y )
	{
		return find( x ) == find( y );
	}

	bool unite( int x, int y )
	{
		x = find( x );
		y = find( y );

		if ( x == y )
		{
			return false;
		}

		if ( rank[x] < rank[y] )
		{
			parent[x] = y;
			sizes[y] += sizes[x];
		}
		else
		{
			parent[y] = x;
			sizes[x] += sizes[y];

			if ( rank[x] == rank[y] )
			{
				++rank[x];
			}
		}

		--groups_;
		return true;
	}

	int groups() const
	{
		return groups_;
	}

	int groupSize( const int x )
	{
		return sizes[ find( x ) ];
	}
};
// UnionFind( N )
// find( x )
// same( x, y )
// unite( x, y )
// groups()
// groupSize( x )

int main()
{
	cin.tie( 0 );
	ios::sync_with_stdio( false );
	cout << setprecision( 12 ) << fixed;

	int N, L;
	scanf( "%d%d", &N, &L );

	VT< LL > A( N );
	for_each( ALL( A ), []( LL &a ){ scanf( "%lld", &a ); } );

	using State = tuple< LL, int, int, int >;
	// ( len, knot, l, r )
	priority_queue< State > que;
	REP( i, N - 1 )
	{
		que.EM( A[i] + A[ i + 1 ], i, i, i + 1 );
	}

	UnionFind uf( N );

	VI res, done( N - 1 ), lefts( N ), rights( N );
	iota( ALL( lefts ), 0 );
	iota( ALL( rights ), 0 );

	while ( !que.empty() )
	{
		const LL len = get< 0 >( que.top() );
		const int k = get< 1 >( que.top() );
		const int l = get< 2 >( que.top() );
		const int r = get< 3 >( que.top() );
		que.pop();

		if ( done[k] )
		{
			continue;
		}

		if ( len < L )
		{
			puts( "Impossible" );
			fflush( stdout );
			return 0;
		}

		res.PB( k + 1 );
		done[k] = 1;

		uf.unite( k, k + 1 );

		const int g = uf.find( k );
		A[g] = len;
		lefts[g] = l;
		rights[g] = r;

		if ( 0 < l )
		{
			que.EM( A[ uf.find( l - 1 ) ] + len, l - 1, lefts[ l - 1 ], rights[g] );
		}
		if ( r < N - 1 )
		{
			que.EM( len + A[ uf.find( r + 1 ) ], r, lefts[g], rights[ r + 1 ] );
		}
	}

	puts( "Possible" );
	for_each( res.rbegin(), res.rend(), []( const int r ){ printf( "%d\n", r ); } );
	fflush( stdout );

	return 0;
}


Measured memory_bytes: 9840