import qualified Data.ByteString.Char8 as BC
import Data.Maybe

readIntsLine :: IO [Int]
readIntsLine = map (fst . fromJust . BC.readInt) . BC.words <$> BC.getLine

comb :: Int -> [a] -> [[a]]
comb 0 xs = [[]]
comb _ [] = []
comb n (x:xs) = [x:y | y <- comb (n-1) xs] ++ comb n xs

main :: IO ()
main = do
  getLine
  ds <- readIntsLine
  let mul [x, y] = x * y
  print . sum . map mul $ comb 2 ds