Aufbau principle states that in the ground state of atom,The electrons are arranged in the order of their increasing energies.
No two electrons will have all four quantum numbers (s,p,d,f) identical.
The first electron will enter the sub shell that has low energy level i.e. s - sub shell from 1s and then 2s and so on.
So while adding electron in an atom the first electron will be added only in 1s.
The further electrons will be added in the order of next sub shells.
Z= 19, Potassium: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1