main = print . sum . map fst . filter (uncurry (<=)) $ sums

sums = map (\x -> (x, sum (divisors x))) candidates
  where candidates = [1..48] ++ [49,51..20161]

divisors n = ds ++ map (n `div`) (init ds)
  where ds = [d | d <- [1..sqrt' n], n `mod` d == 0]
        sqrt' = truncate . sqrt . fromIntegral