module Main where
import qualified Control.Monad as C
import qualified Data.Char as C
import qualified Data.List as L
import qualified Data.Map as M
import qualified Data.Array as A

int = read :: String -> Int
getWord :: IO String
getWord = do
  c <- getChar
  if C.isSpace c
  then
    return []
  else do
    ws <- getWord
    return (c:ws)
getInt = fmap int getWord
replace pred a = map (\x -> if pred x then a else x)

main = interact $ printer . solver . parser
parser = map int . words
printer = unlines . (:[]) . show

solver (n:l:ss) = sum apples - a
  where
    apples = [l+i-1|i<-[1..n]]
    (m,a) = minimum $ zip (map abs apples) apples