main = getLine >>= print . solve . map read . words

solve :: [Int] -> Int
solve [a, b, c, d] = f a b d where
    f y n m
        | y == 0        = 0
        | n < c         = 0
        | m - c - 1 < 0 = 0
        | otherwise     = 1 + f (y - 1) (n - c) (m - c - 1)