import Control.Monad
 
import Data.Array.IO
 

yoba :: Int -> IO Int  
yoba n = do arr <- newArray_ (0, n) :: IO (IOUArray Int Int)
            writeArray arr 0 0                                             
            forM_ [1..n] (\v -> readArray arr (v - 1) >>= readArray arr >>= writeArray arr v . (v -))
            readArray arr n
 
main = yoba 50000000 >>= print