import Control.Applicative

calc :: IO(Integer) -> IO([Integer]) -> IO(Integer)
calc acc io = do
  a <- acc
  [c,d] <- io
  return (a+(c+1)`div`2*d)

main :: IO()
main = do
  n <- (read::String->Int) <$> getLine
  let l = replicate n ((fmap (read::String->Integer) . words) <$> getLine)
  print =<< foldl calc (return 0) l