xs = [ 8,2,22,97,38,15,0,40,0,75,4,5,7,78,52,12,50,77,91,8,
       49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,4,56,62,0,
       81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,3,49,13,36,65,
       52,70,95,23,4,60,11,42,69,24,68,56,1,32,56,71,37,2,36,91,
       22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80,
       24,47,32,60,99,3,45,2,44,75,33,53,78,36,84,20,35,17,12,50,
       32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70,
       67,26,20,68,2,62,12,20,95,63,94,39,63,8,40,91,66,49,94,21,
       24,55,58,5,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72,
       21,36,23,9,75,0,76,44,20,45,35,14,0,61,33,97,34,31,33,95,
       78,17,53,28,22,75,31,67,15,94,3,80,4,62,16,14,9,53,56,92,
       16,39,5,42,96,35,31,47,55,58,88,24,0,17,54,24,36,29,85,57,
       86,56,0,48,35,71,89,7,5,44,44,37,44,60,21,58,51,54,17,58,
       19,80,81,68,5,94,47,69,28,73,92,13,86,52,17,77,4,89,55,40,
       4,52,8,83,97,35,99,16,7,97,57,32,16,26,26,79,33,27,98,66,
       88,36,68,87,57,62,20,72,3,46,33,67,46,55,12,32,63,93,53,69,
       4,42,16,73,38,25,39,11,24,94,72,18,8,46,29,32,40,62,76,36,
       20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,4,36,16,
       20,73,35,29,78,31,90,1,74,31,49,71,48,86,81,16,23,57,5,54,
       1,70,54,71,83,51,54,69,16,92,33,48,61,43,52,1,89,19,67,48 ]

allSeqOfLength :: Int -> Int -> Int -> [a] -> [[a]]
allSeqOfLength n w h xs = 
    horizontal ++ vertical ++ forwardDiag ++ backDiag
  where
    horizontal   = runs [0..w-n]   [0..h-1] 1
    vertical     = runs [0..w-1]   [0..h-n] w
    forwardDiag  = runs [n-1..w-1] [0..h-n] (w-1)
    backDiag     = runs [0..w-n]   [0..h-n] (w+1)
    runs xs ys s = [run (x+y*w) s | x <- xs, y <- ys]
    run i s      = map (xs!!) [i,i+s..i+(n-1)*s]

answer = maximum $ map (product) $ allSeqOfLength 4 20 20 xs
