import Control.Monad

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