Visegrad Group (V4) is an unformal alliance of four central-european states: Poland, Czech Republic, Slovakia and Hungary.
Try to name 15 biggest cities of V4, cell color can help you ;)
Name 15 biggest cities of the Visegrad Group
Based on: citypopulation.de
See also: https://en.wikipedia.org/wiki/Visegr%C3%A1d_Group