import Control.Applicative ((<$>))
import Control.Monad (replicateM)
import Data.List

main :: IO ()
main = do
  n <- readLn
  solve <$> replicateM (n - 1) readLn <*> replicateM n (getl $ map read . words) >>= print

solve :: [Int] -> [[Int]] -> Int
solve as bcs = loop 0 0 (0:as) bcs
  where
    loop fr pn [] [] = fr
    loop fr pn (a:xs) ([b, c]:ys) = loop (fr + pn * a) (pn - b + c) xs ys

getl :: (String -> a) -> IO a
getl f = f <$> getLine