module Main where

factorial :: Integer -> Either String Integer
factorial n = if n < 0 then 
    Left "n cannot be negative" 
    else Right $ factorial' n 1 where 
        factorial' 0 res = res
        factorial' n res = factorial' (n - 1) $! n * res

main :: IO()
main = do
    s <- getLine
    let n = (read s :: Integer) 
    let res = factorial n
    case res of
        Right r -> do 
            print r
        Left err -> do
            putStrLn $ "Error: " ++ err