No wait: there are periods after both R. and P. This means it is "Reka Pivdenniyi (abbreviated) "Bug", or "River Southern Bug".

Probably it says "River Bug" (pronounced boog) in Ukrainian. Russian would be "reka" and would not abbreviate to "rp" but I don't know what it is in Ukrainian dialect. There are two rivers with this name, one on the border with Poland and one that flows past Vinnitsa into the Black Sea. See wikipedia for short descriptions.