>The way we learned to count syllables was "count the vowels". Or, if you're not sure, try to sing the word, one syllable - one note. Including the funny cases where the song requires the singer to extend the r-as-vowel (which actually happens quite often - drvo (tree), srce (heart), prvi (first), rđa (rust)... and, surprise surprise, Hrvatska (Croatia) and Srbija (Serbia) :).

You wouldn't have to if you didn't loose vowels in those words in the first place. :)
In Russian: derevo, serdce, perviy, ruda.