main :: IO ()
main = do
  [h, w, a, b] <- map read . words <$> getLine
  print $ sum [combi (h-a-1-b+i) (b+i) + combi (w-b-1+a+i) (a+1) | i <- [1..(w-b)]]

combi :: (Integral a) => a -> a -> a
combi n m = fact' n (m-1) `div` fact m

fact :: (Integral a) => a -> a
fact n = if n == 0 then 1 else n * fact (n-1)

fact' :: (Integral a) => a -> a -> a
fact' n m = if m == 0 then n else (n-m) * fact' n (m-1)
