def main():

    N, L = (int(i) for i in input().split())

    A = [int(i) for i in input().split()]



    ma = 0

    idx = -1

    for i in range(N-1):

        if ma < A[i] + A[i+1]:

            ma = A[i] + A[i+1]

            idx = i+1

    if ma < L:

        return print("Impossible")

    print("Possible")

    for i in range(1, idx):

        print(i)

    for i in range(idx, N)[::-1]:

        print(i)





if __name__ == '__main__':

    main()

def main():

    import sys

    input = sys.stdin.buffer.readline

    N, L = (int(i) for i in input().split())

    A = [int(i) for i in input().split()]

    last = -1

    for i in range(N-1):

        if L <= A[i] + A[i+1]:

            last = i+1

    if last < 0:

        return print("Impossible")

    ans = [i for i in range(1, last)] + \

        [i for i in range(last+1, N)][::-1] + [last]

    print("Possible")

    print(*ans, sep="\n")





if __name__ == '__main__':

    main()

def main():
    N, L = (int(i) for i in input().split())
    A = [int(i) for i in input().split()]
    ma = 0
    idx = -1
    for i in range(N - 1):
        if ma < A[i] + A[i + 1]:
            ma = A[i] + A[i + 1]
            idx = i + 1
    if ma < L:
        return print("Impossible")
    print("Possible")
    for i in range(1, idx):
        print(i)
    for i in range(idx, N)[::-1]:
        print(i)


if __name__ == "__main__":
    main()

def main():
    import sys

    input = sys.stdin.buffer.readline
    N, L = (int(i) for i in input().split())
    A = [int(i) for i in input().split()]
    last = -1
    for i in range(N - 1):
        if L <= A[i] + A[i + 1]:
            last = i + 1
    if last < 0:
        return print("Impossible")
    ans = [i for i in range(1, last)] + [i for i in range(last + 1, N)][::-1] + [last]
    print("Possible")
    print(*ans, sep="\n")


if __name__ == "__main__":
    main()

+    import sys + +    input = sys.stdin.buffer.readline -    ma = 0 -    idx = -1 +    last = -1 -        if ma < A[i] + A[i + 1]: -            ma = A[i] + A[i + 1] -            idx = i + 1 -    if ma < L: +        if L <= A[i] + A[i + 1]: +            last = i + 1 +    if last < 0: +    ans = [i for i in range(1, last)] + [i for i in range(last + 1, N)][::-1] + [last] -    for i in range(1, idx): -        print(i) -    for i in range(idx, N)[::-1]: -        print(i) +    print(*ans, sep="\n")