Where does “полустолетие” come from?

полустолетие (Russian) comes from Russian столетие, from Bulgarian сто, from Old Church Slavonic съто, from Proto-Slavic sъto, from Proto-Balto-Slavic śímta, from Proto-Indo-European ḱm̥tóm, from Proto-Indo-European déḱm̥ — ten.

Ancestry of “полустолетие”, step by step

StepLanguageWordMeaning
1Russianстолетиеcentury, hundred-year period; centenary,...
2Bulgarianстоhundred
3Old Church Slavonicсътоhundred
4Proto-Slavicsъtohundred
5Proto-Balto-Slavicśímtahundred
6Proto-Indo-Europeanḱm̥tómhundred
7Proto-Indo-Europeandéḱm̥ten
Every word from Proto-Indo-European déḱm̥ →Every word from Proto-Indo-European ḱm̥tóm →Every word from Proto-Balto-Slavic śímta →