{-# LANGUAGE BangPatterns, OverloadedStrings #-}
import qualified Data.ByteString.Char8 as BS

main = do
    [a,b] <- map ((read :: String -> Int) . BS.unpack) . BS.words <$> BS.getLine
    case headSafe [ 10*b+l | l <- [0..9], floor ((fromIntegral (10*b + l))*0.08) == a] of
        Just x -> print x
        Nothing -> print (-1)

headSafe :: [a] -> Maybe a
headSafe (x:xs) = Just x
headSafe [] = Nothing