import Data.List
import Control.Applicative
import Debug.Trace

main :: IO ()
main = solve <$> (map read . words <$> getLine) <*> (map (map read . words) . lines <$> getContents) >>= putStrLn . yesno

yesno :: Bool -> String
yesno True = "YES"
yesno False = "NO"

solve :: [Int] -> [[Int]] -> Bool
solve [n, m] [] = True
solve [n, m] g = let deg = (map length . group . sort . concat) g
                 in if (all even) deg || (length . filter odd) deg == 2
                      then let reachable = dfs ((head . head) g) [] g
                           in (sort . nub) reachable == (sort . nub . concat) g
                      else False

dfs :: Int -> [Int] -> [[Int]] -> [Int]
dfs u vis g = foldl' f (u:vis) (adj1 ++ adj2)
  where
    adj1 = map last . filter ((==u) . head) $ g
    adj2 = map head . filter ((==u) . last) $ g

    f :: [Int] -> Int -> [Int]
    f vis v
      | v `elem` vis = vis
      | otherwise    = dfs v vis g