python將1234倒序輸出4321，-1234輸出-4321

try:
    a = input("請輸入一個數：")
    if int(a) > 0:
        b = a[::-1]
        print(b)
    else:
        b = "-"+ a[-1:-len(a):-1]
        print(b)
except Exception as e:
    print("輸入的為非數字")