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import System.Environment
import Control.Monad
import Control.Concurrent
main = do
[n] <- map read <$> getArgs
mvars <- replicateM n newEmptyMVar
sequence_ [ forkIO $ putMVar m $! nsoln n
| (m,n) <- zip mvars (repeat 9) ]
mapM_ takeMVar mvars
nsoln nq = length (gen nq)
where
safe :: Int -> Int -> [Int] -> Bool
safe x d [] = True
safe x d (q:l) = x /= q && x /= q+d && x /= q-d && safe x (d+1) l
gen :: Int -> [[Int]]
gen 0 = [[]]
gen n = [ (q:b) | b <- gen (n-1), q <- [1..nq], safe q 1 b]
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