import Data.Maybe
import Data.IORef
import qualified Data.ByteString.Char8 as B

main :: IO ()
main = do
  f <- intReader
  [n] <- f 1
  xs <- sequence . replicate n $ f 2 :: IO [[Int]]
  let a = head . head $ xs
  let b = last . head $ xs
  putStrLn $ if (all (== a) . map head $ xs) || (all (== b) . map last $ xs) then "YES" else "NO"

intReader :: IO (Int -> IO [Int])
intReader = do
  ws <- fmap ((concatMap B.words) . B.lines) B.getContents >>= newIORef
  return $ \n -> do
    xs <- readIORef ws
    writeIORef ws (drop n xs)
    return (take n . map (fst . fromJust . B.readInt) $ xs)