import Control.Monad

main = do
  n <- readLn :: IO Int
  cs <- map (map read . words) <$> replicateM n getLine :: IO [[Int]]
  putStrLn . show $ sum $ map fn cs
  where
    fn [x, y] = if even x then x `div` 2 * y else (x `div` 2 + 1) * y