Problem 9.5. On an island, there live knights who always tell the truth, and liars who always lie.

One day, 80 residents of the island gathered, each wearing a T-shirt with a number from 1 to 80 (different residents had different numbers). Each of them said one of two phrases:

- "Among those gathered, at least 5 liars have a T-shirt number greater than mine."
- "Among those gathered, at least 5 liars have a T-shirt number less than mine."

What is the smallest number of knights that could have been among these 80 residents?