{- Karol Samborski -}
module Main where

import Data.List
import Data.Array

type Edge   = Int
data Vertex = Vertex { nr :: Int, edges :: [Edge], low :: Int }

type Bridge = (Int, Int)

findBridges :: Array Int Vertex -> [Bridge]
findBridges vs = foldr (\(i, v) acc ->
    if low v == nr v
        then (zip [i,i..] $ filter (\e -> low (newVs!e) /= low v) $ edges v) ++ acc
        else acc) [] $ assocs newVs
  where
    newVs = dfsBridges [1] vs 1

dfsBridges :: [Int] -> Array Int Vertex -> Int -> Array Int Vertex
dfsBridges []     ar _ = ar
dfsBridges (v:vs) ar d = dfsArray // [(v, (dfsArray!v) { low = foldr (\e l ->
    if (nr $ dfsArray!e) > (-1) && e /= (d-1) && (low $ dfsArray!e) < l
        then (low $ dfsArray!e)
        else l) d $ edges $ dfsArray!v})]
  where
    initArr = ar//[(v, (ar!v) {nr = d, low = d})]
    stack = foldr (\e l ->
        if (nr $ initArr!e) > (-1)
            then l
            else l ++ [e]) vs $ edges $ ar!v
    dfsArray = dfsBridges stack initArr (d+1)

main :: IO ()
main = do
    count <- (getLine >>= readIO) :: IO Int
    v <- mapM (\i -> do
        l <- (getLine >>= readIO) :: IO [Int]
        return $ Vertex (-1) l (-1)) [1..count]
    putStrLn $ show $ findBridges $ listArray (1,count) v