In the storage calculation of this problem, it is stated that:

**Storage estimates:** Let’s assume we store every URL shortening request (and associated shortened link) for 5 years. Since we expect to have 500M new URLs every month, the total number of objects we expect to store will be 30 billion:

*500 million * 5 years * 12 months = 30 billion*

So that would be 30B URLs in 5 years.

But then, when talking about encoding the original URL, it says:

*With 68.7B unique strings, let’s assume six letter keys would suffice for our system.*

So the system would be able to run for only ~10 years? Because after 10 years we would have used ~60B unique short URLs.

Thank you in advance.