{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE BangPatterns          #-}
{-# LANGUAGE CPP                   #-}
{-# LANGUAGE LambdaCase            #-}
{-# LANGUAGE MagicHash             #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE MultiWayIf            #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE TupleSections         #-}
{-# LANGUAGE TypeFamilies          #-}
{-# LANGUAGE ViewPatterns          #-}

import           Control.Applicative
import           Control.Exception
import           Control.Monad
import           Control.Monad.Primitive
import           Control.Monad.ST
import           Control.Monad.State.Strict
import           Data.Bool
import qualified Data.ByteString             as B
import qualified Data.ByteString.Builder     as B
import qualified Data.ByteString.Char8       as C
import qualified Data.ByteString.Internal    as B
import qualified Data.ByteString.Unsafe      as B
import           Data.Char
import qualified Data.Foldable               as F
import           Data.Function
import qualified Data.IntMap.Strict          as IM
import qualified Data.IntSet                 as IS
import qualified Data.List                   as L
import qualified Data.Map.Strict             as M
import           Data.Monoid
import           Data.Ord
import           Data.Primitive.MutVar
import qualified Data.Set                    as S
import           Data.Tuple
import qualified Data.Vector                 as V
import qualified Data.Vector.Mutable         as VM
import qualified Data.Vector.Generic         as G
import qualified Data.Vector.Generic.Mutable as GM
import qualified Data.Vector.Unboxed         as U
import qualified Data.Vector.Unboxed.Mutable as UM
import           Debug.Trace
import           Foreign                     hiding (void)
import           GHC.Exts
import qualified System.IO                   as IO
import           Unsafe.Coerce

main :: IO ()
main = do
    !n <- readLn :: IO Int
    mat <- U.unfoldrN (n*(n-1)) parseInt <$> C.getContents
    print . maybe (-1) id $ solve n $ V.generate n $ \i -> U.slice (i * (n - 1)) (n - 1) mat

type TaskId = Int

taskId :: Int -> Int -> TaskId
taskId x y = min x y * 1024 + max x y

solve :: Int -> V.Vector (U.Vector Int) -> Maybe Int
solve n (V.imap (U.map . taskId . (+1)) -> priority0) = runST $ do
    next <- VM.replicate (1024 * 1024) []
    inDeg <- UM.replicate (1024 * 1024) (0 :: Int)
    V.forM_ priority0 $ \row -> do
        U.forM_ (U.zip row $ U.tail row) $ \(v, nv) -> do
            VM.unsafeModify next (nv:) v
            UM.unsafeModify inDeg (+1) nv
    queue <- newQueueM
    tasks0 <- V.filterM (\p -> (== 0) <$> UM.unsafeRead inDeg p)
        (V.map U.unsafeHead priority0)
    forM_ (IS.toList $ V.foldl' (flip IS.insert) IS.empty tasks0) $ \p -> do
        d <- UM.unsafeRead inDeg p
        when (d == 0) $ do
            enqueueM p queue
    flip fix 0 $ \loop !i -> do
        tasks <- runVecQueue queue
        -- traceShowM ("out", tasks)
        if U.null tasks
        then do
            count <- UM.read (queueInfo queue) 1
            -- traceShowM ("count", count)
            if count == n * (n - 1) `div` 2
            then return $ Just i
            else return Nothing
        else do
            U.forM_ tasks $ \task -> do
                childlen <- VM.unsafeRead next task
                forM_ childlen $ \child -> do
                    d <- UM.unsafeRead inDeg child
                    UM.unsafeWrite inDeg child (d - 1)
                    when (d == 1) $ do
                        enqueueM child queue
                        -- traceShowM ("in", child)
            loop (i + 1)

runVecQueue :: (PrimMonad m, UM.Unbox a) => (VecQueue (PrimState m) a) -> m (U.Vector a)
runVecQueue (VecQueue info q) = do
    h <- UM.unsafeRead info 0
    t <- UM.unsafeRead info 1
    UM.unsafeWrite info 0 t
    U.freeze (UM.slice h (t - h) q)

-------------------------------------------------------------------------------

data VecQueue m a = VecQueue
    { queueInfo :: !(UM.MVector m Int)
    , queueData :: !(UM.MVector m a)
    }

newQueueM :: (PrimMonad m, UM.Unbox a) => m (VecQueue (PrimState m) a)
newQueueM = VecQueue <$> UM.replicate 2 0 <*> UM.unsafeNew (1024 * 1024)

dequeueM :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m (Maybe a)
dequeueM (VecQueue info q) = do
    h <- UM.unsafeRead info 0
    t <- UM.unsafeRead info 1
    if h < t
    then do
        UM.unsafeWrite info 0 (h + 1)
        pure <$> UM.unsafeRead q h
    else return Nothing
{-# INLINE dequeueM #-}

enqueueM :: (PrimMonad m, UM.Unbox a) => a -> VecQueue (PrimState m) a -> m ()
enqueueM x (VecQueue info q) = do
    t <- UM.unsafeRead info 1
    UM.unsafeWrite q t x
    UM.unsafeWrite info 1 (t + 1)
{-# INLINE enqueueM #-}


type Parser a = C.ByteString -> Maybe (a, C.ByteString)

parseInt :: Parser Int
parseInt = C.readInt . C.dropWhile isSpace

parseInt2 :: Parser (Int, Int)
parseInt2 = runStateT $
    (,) <$> StateT (C.readInt . C.dropWhile isSpace)
        <*> StateT (C.readInt . B.unsafeTail)

parseInt3 :: Parser (Int, Int, Int)
parseInt3 = runStateT $
    (,,) <$> StateT (C.readInt . C.dropWhile isSpace)
        <*> StateT (C.readInt . B.unsafeTail)
        <*> StateT (C.readInt . B.unsafeTail)

parseInt4 :: Parser (Int, Int, Int, Int)
parseInt4 = runStateT $
    (,,,) <$> StateT (C.readInt . C.dropWhile isSpace)
        <*> StateT (C.readInt . B.unsafeTail)
        <*> StateT (C.readInt . B.unsafeTail)
        <*> StateT (C.readInt . B.unsafeTail)

Measured memory_bytes: 95612