getSmall x y = if x < y then x else y

solve [x,y,d]
    | d == 0            = 1
    | x < d && y < d    = 0
    | x' == y'          = x' + 1
    | otherwise         = ans + 1
    where
        x' = getSmall x d
        y' = getSmall y d
        ans = getSmall x' y'

main = do
    xyd <- map (abs . read :: String -> Int) . words <$> getLine
    print $ solve xyd