Pink Floyd 10 - WHAT SHALL WE DO NOW Lyrics

[backwards message:]
"Congratulations, You have just discovered the secret message.
Please send your answer to 'Old Pink',
Care of the funny farm, Chalfont..." - Spoken By Roger
[...interrupted...]
"Roger, Caroline's on the phone..." - Unknown Voice

What shall we use
To fill the empty s___es
Where we used to talk?
How shall I fill
The final places?
How should I complete the wall