toHHMM :: String -> (Int, Int)
toHHMM s = ((read :: String -> Int) . takeWhile (/= ':') $ s, (read :: String -> Int) . tail . dropWhile (/= ':') $ s)

sleepTime :: (Int, Int) -> (Int, Int) -> Int
sleepTime (h1, m1) (h2, m2)
    | h2 >  h1 && m2 >= m1 = (h2 - h1) * 60 + m2 - m1
    | h2 >  h1 && m2 <  m1 = (h2 - 1 - h1) * 60 + m2 + 60 - m1
    | h2 == h1 && m2 >= m1 = m2 - m1
    | otherwise            = (h2 + 23 - h1) * 60 + m2 + 60 - m1

splitAt2 :: [String] -> [[String]]
splitAt2 = takeWhile (not.null) . map (take 2) . iterate (drop 2)

solve :: [[String]] -> Int
solve xss = sum zs
    where
        ys = map (map toHHMM) xss
        zs = map (\[x, y] -> sleepTime x y) ys

main :: IO ()
main = interact $ show . solve . splitAt2 . tail . words