import Control.Applicative

main :: IO ()
main = solve <$> f >>= print
  where
    f = map read <$> words <$> getLine

solve :: [Int] -> Int
solve [a, b, c]
  | z >= 1 = z
  | otherwise = -1
  where
    x = 3600 * c
    y = 60 * a - b
    z = (x + y - 1) `div` y