// dfmt off
T lread(T=long)(){return readln.chomp.to!T;}T[] lreads(T=long)(long n){return iota(n).map!((_)=>lread!T).array;}
T[] aryread(T=long)(){return readln.split.to!(T[]);}void arywrite(T)(T a){a.map!text.join(' ').writeln;}
void scan(L...)(ref L A){auto l=readln.split;foreach(i,T;L){A[i]=l[i].to!T;}}alias sread=()=>readln.chomp();
void dprint(L...)(lazy L A){debug{auto l=new string[](L.length);static foreach(i,a;A)l[i]=a.text;arywrite(l);}}
static immutable MOD=10^^9+7;alias PQueue(T,alias l="b<a")=BinaryHeap!(Array!T,l);import std, core.bitop;
// dfmt on

void main()
{
    long N, L;
    scan(N, L);
    auto A = aryread();
    foreach (i; 0 .. N - 1)
    {
        if (L <= A[i] + A[i + 1])
        {
            writeln("Possible");
            auto ans = new long[](N - 1);
            long ptr;
            ans[ptr++] = i;
            foreach (j; i + 1 .. N - 1)
            {
                ans[ptr++] = j;
            }
            foreach_reverse (j; 0 .. i)
            {
                ans[ptr++] = j;
            }
            foreach_reverse (a; ans)
            {
                writeln(a + 1);
            }
            return;
        }
    }
    writeln("Impossible");
}
