main :: IO ()
main = readLn >>= print . solve

solve :: Int -> Int
solve a = f b + f c
    where
        b = (floor ( fromIntegral a / fromIntegral 2 )) - 1
        c = (ceiling ( fromIntegral a / fromIntegral 2 )) - 1
        f :: Int -> Int
        f x = x * (x + 1) `div` 2