Twos Complement to Denary

twos = input("Please Enter an 8 digit Twos Complement Number")
if len(twos)!=8:
    print("Invalid length")
    #return
print(len(twos))
for i in twos:
        if int(i) > 1:
            print("invalid value")
            #return
if twos[0]=="1":
    print("negative twos complement")
    twos = int(twos, 2)
    decimal = 256 - twos
    print("decimal: -",decimal)
    hexa=(hex(decimal))
    print("Hex: -",hexa[2::])
else:
    print("positive twos complement")
    number = int(twos,2)
    print("decimal:",number)
    hexa=(hex(number))
    print("Hex:",hexa[2::])