import std.stdio, std.string, std.conv;
import std.range, std.algorithm, std.array, std.typecons;
import std.math, std.numeric;

void main() {
    int N, L; scan(N, L);
    auto a = readln.split.to!(int[]);

    int r = -1;
    foreach (i ; 1 .. N) {
        if (a[i] + a[i-1] >= L) {
            r = i;
            break;
        }
    }
    if (r == -1) {
        writeln("Impossible");
        return;
    }
    writeln("Possible");
    
    foreach_reverse (i ; r + 1 .. N) {
        writeln(i);
    }
    foreach (i ; 1 .. r) {
        writeln(i);
    }
    writeln(r);
}






void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}



void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}
