toInt :: String -> Int
toInt = read :: String -> Int

calcTrapezoidArea :: Int -> Int -> Int -> Int
calcTrapezoidArea a b h = (a + b) * (h `div` 2)

main = do
  a <- getLine
  b <- getLine
  h <- getLine
  putStrLn $ show $ calcTrapezoidArea (toInt a) (toInt b) (toInt h)