Read carefully through all the hints provided (some solutions submitted during the 4th week failed to notice some recommendations provided by the 4th hint). Also, you are welcome to use formulae derived by the solution of the 3rd problem of the EuPhO-2018, in particular, there is a useful formula for the energy change \epsilon_n(\delta) due to a small displacement \delta of the n(\delta)-th step, which appears to be independent of the step index number n(\delta).
Submission of the solutions of the Problem 2 will remain open at least until 26th January, 14:00 GMT. If the total number of correct solutions from university students remains still smaller than 10, the deadline will be extended one more time, until 2nd February, 14:00 GMT. However, no more hints will be provided.