Complex sentence: Ukrainians during the interbellum had several representatives in the Sejm (Polish parliament), in 1928-1930 there were 26 of them, including Marshall Deputy of the Sejm, Volodymyr Zahajkiewicz and the Secretary of the Sejm, Dymitr Ladyka.
Split into simpler sentences: Ukrainians during the interbellum had several representatives in the Sejm (Polish parliament). In 1928--1930 there were 26 of them, including Marshall Deputy of the Sejm, Volodymyr Zahajkiewicz and the Secretary of the Sejm, Dymitr Ladyka.