import sys

input = sys.stdin.readline

N, L = list(map(int, input().split()))

a = list(map(int, input().split()))

s = -1

for i in range(N - 1):

  if a[i] + a[i + 1] >= L:

    s = i

    break

if s == -1:

  print("Impossible")

  exit(0)

res = [s]

for i in range(s - 1, -1, -1):

  res.append(i)

for i in range(s + 1, N - 1):

  res.append(i)

res.reverse()

print("Possible")

for r in res: print((r + 1))
import sys

input = sys.stdin.readline

N, L = list(map(int, input().split()))

a = list(map(int, input().split()))

res = []

for i in range(N - 1):

  if a[i] + a[i + 1] >= L:

    res.append(i)

    break

else:

  print("Impossible")

  exit(0)

for i in range(res[0] - 1, -1, -1): res.append(i)

for i in range(res[0] + 1, N - 1): res.append(i)

res.reverse()

print("Possible")

for r in res: print((r + 1))
import sys

input = sys.stdin.readline
N, L = list(map(int, input().split()))
a = list(map(int, input().split()))
s = -1
for i in range(N - 1):
    if a[i] + a[i + 1] >= L:
        s = i
        break
if s == -1:
    print("Impossible")
    exit(0)
res = [s]
for i in range(s - 1, -1, -1):
    res.append(i)
for i in range(s + 1, N - 1):
    res.append(i)
res.reverse()
print("Possible")
for r in res:
    print((r + 1))

import sys

input = sys.stdin.readline
N, L = list(map(int, input().split()))
a = list(map(int, input().split()))
res = []
for i in range(N - 1):
    if a[i] + a[i + 1] >= L:
        res.append(i)
        break
else:
    print("Impossible")
    exit(0)
for i in range(res[0] - 1, -1, -1):
    res.append(i)
for i in range(res[0] + 1, N - 1):
    res.append(i)
res.reverse()
print("Possible")
for r in res:
    print((r + 1))

-s = -1 +res = [] -        s = i +        res.append(i) -if s == -1: +else: -res = [s] -for i in range(s - 1, -1, -1): +for i in range(res[0] - 1, -1, -1): -for i in range(s + 1, N - 1): +for i in range(res[0] + 1, N - 1):