Replace the question mark with a number

On wife's b'day, man ordered a cake on phone.
Salesman: What message to put on the cake?
Man: Write “Getting older but U R getting better.”
Salesman: kaise likhna hai message ?
Man: Well.. put “U R getting older” at the top and “but U R getting better” at the bottom.
When the cake was opened all guests died laughing at the message.
It read: “You are getting older at the top, but you are getting better at the bottom!”