{-# 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

    fix `flip` 0 `flip` priority0 $ \loop !i !prios -> do
        if i < 1024 * 1024
        then do
            if V.null prios
            then return $ Just i
            else do
                tasks <- V.foldl' (flip $ IS.insert . U.unsafeHead) IS.empty
                    <$> V.filterM (\p -> (== 0) <$> UM.unsafeRead inDeg (U.unsafeHead p)) prios
                forM_ (IS.toList tasks) $ \task -> do
                    children <- VM.unsafeRead next task
                    forM_ children $ \child -> do
                        UM.unsafeModify inDeg (subtract 1) child
                loop (i + 1)
                    . V.filter (not.U.null)
                    $ V.map (U.dropWhile (`IS.member` tasks)) prios
        else return Nothing

-------------------------------------------------------------------------------
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)