2020 IMO Problems/Problem 4
There is an integer . There are stations on a slope of a mountain, all at different altitudes. Each of two cable car companies, and , operates cable cars; each cable car provides a transfer from one of the stations to a higher one (with no intermediate stops). The cable cars of have different starting points and different finishing points, and a cable car that starts higher also finishes higher. The same conditions hold for . We say that two stations are linked by a company if one can start from the lower station and reach the higher one by using one or more cars of that company (no other movements between stations are allowed).
Determine the smallest positive integer k for which one can guarantee that there are two stations that are linked by both companies.
https://www.youtube.com/watch?v=dTqwOoSfaAA [video covers all day 2 problems]