Barber

A cheeky young lad after having his hair cut turned to the barber and said, 'Right, I'll have my ice-cream now.'

'What do you mean?' said the barber.
'Well you have a sign at the front window which reads
What do you think? I'll cut your hair for nothing and buy buy you an ice-cream!'

'Oh n! replied the barber. 'My silly assistant put the punctuation marks in the wrong place.'
Can you explain how the barber should have punctuated the sign?