let rec sumWays x y n =
    if x = n && y = n
        then 0
    else if x = n
        then 1 + (sumWays x (y + 1) n)
    else if y = n
        then 1 + (sumWays (x + 1) y n)
    else
        2 + (sumWays (x + 1) y n) + (sumWays x (y + 1) n)

sumWays 0 0 2