Potassium negatively charged?

A question states:

Particle X contains 9 protons, 10 neutrons, and 9 electrons. Particle Y contains 9 protons, 10 neutrons, and 10 electrons. What is the relationship between particles X and Y.


I said that they were both potassium due to their atomic mass being 19. The issue is particle Y has one extra electron which suggests that it’s negatively charged. As far as I can tell, K^- doesn’t exist naturally.

Perhaps, this is an error in the question? Or, have I misunderstood?

Potassium indeed has atomic number 19, BUT this is NOT the mass number.
These particles are fluorine (atomic number 9)