Help! Make the ff code work.

Question

##### Example Code 1

ll = BSCS('H').add('e').add('l').add('l').add('o').add('World!')
print(ll.sum())

##### Output for Example Code 1

Hello

##### Example Code 2

ll1 = BSCS(1).add(2).add(3).add(4).add(5)
print(ll1.sum())

##### Output for Example Code 1

15