Submission #2073663

---

Source Code Expand

import Control.Monad

main = do
  [n,d] <- map read . words <$> getLine :: IO [Integer]
  [x,y] <- map read . words <$> getLine :: IO [Integer]
  (print . maybe 0 id) (jump n d x y)

jump n d x0 y0 = do
  let (x,rx) = divMod x0 d
  let (y,ry) = divMod y0 d
  guard (rx == 0 && ry == 0)
  let c = sum [comb i rx + comb (n-i) ry | i <- [x..n], even i == even x, even (n-i) == even y, let rx = div (i+x) 2, let ry = div ((n-i)+y) 2]
  pure (fromIntegral c / fromIntegral (4^n))

comb n r = div (fact n) (fact r * fact (n-r))
  where fact x = product [1..x]

Submission Info

Judge Result