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In page Ukrainian language:

"

Ukrainian case endings are somewhat different from Old East Slavic, and the vocabulary includes a large overlay of Polish terminology. Russian na pervom etaže 'on the first floor' is in the locative (prepositional) case. The Ukrainian corresponding expression is na peršomu poversi (на першому поверсі). -omu is the standard locative (prepositional) ending, but variants in -im are common in dialect and poetry, and allowed by the standards bodies. The kh of Ukrainian poverkh (поверх) has mutated into s under the influence of the soft vowel i (k is similarly mutable into c in final positions).[citation needed]