import Control.Monad
import Data.Maybe
import qualified Data.ByteString.Char8 as B

main = do
 n <- read <$> getLine
 cds <- replicateM n (map (fst . fromJust . B.readInteger) . B.words <$> B.getLine)
 print (proctor cds)

proctor = flip mod m . sum . map (\[c,d] -> div ((mod c m) + 1) 2 * (mod d m))
 where m = 10^9+7