module Main where
import Control.Monad
import Control.Applicative
import Data.Maybe
import Data.List
import qualified Text.Printf
import qualified Data.ByteString.Char8 as BC
import qualified Data.ByteString as BC

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main :: IO ()
main = do
  n <- readInt
  fs <- replicateM (n-1) readInt
  (_,c0):xs <- replicateM n $ do
    [b, c] <- readInts
    return (b, c)
  let h (a,p) (f,(b,c)) = (a+f*p,p-b+c)
  print $ fst $ foldl' h (0,c0) (zip fs xs)

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{- Int input -}

parseInt :: BC.ByteString -> Int
parseInt = fst . fromJust . BC.readInt

parseInts :: BC.ByteString -> [Int]
parseInts = map parseInt <$> BC.words

readInt :: IO Int
readInt = parseInt <$> BC.getLine

readInts :: IO [Int]
readInts = parseInts <$> BC.getLine

{- Double Formatting -}

doubleFmt :: Double -> String
doubleFmt = Text.Printf.printf "%.12f"