stock = [1, 2, 3, 4, 5]
target = [4, 3, 5, 2, 1]

stack = [] 
index = 0  

for book in stock:
    stack.append(book)
    while stack and stack[-1] == target[index]:
        stack.pop()
        index += 1

if index == len(target):
    print("Possible")
else:
    print("Impossible")