Problem:
Score : 500 points

Problem Statement

N players will participate in a tennis tournament. We will call them Player 1, Player 2, \ldots, Player N.

The tournament is round-robin format, and there will be N(N-1)/2 matches in total.
Is it possible to schedule these matches so that all of the following conditions are satisfied? If the answer is yes, also find the minimum number of days required.

Each player plays at most one matches in a day.

Each player i (1 \leq i \leq N) plays one match against Player A_{i, 1}, A_{i, 2}, \ldots, A_{i, N-1} in this order.

Constraints

3 \leq N \leq 1000

1 \leq A_{i, j} \leq N

A_{i, j} \neq i

A_{i, 1}, A_{i, 2}, \ldots, A_{i, N-1} are all different.

Input

Input is given from Standard Input in the following format:

N
A_{1, 1} A_{1, 2} \ldots A_{1, N-1}
A_{2, 1} A_{2, 2} \ldots A_{2, N-1}
:
A_{N, 1} A_{N, 2} \ldots A_{N, N-1}

Output

If it is possible to schedule all the matches so that all of the conditions are satisfied, print the minimum number of days required; if it is impossible, print -1.

Sample Input 1

3
2 3
1 3
1 2

Sample Output 1

3

All the conditions can be satisfied if the matches are scheduled for three days as follows:

Day 1: Player 1 vs Player 2

Day 2: Player 1 vs Player 3

Day 3: Player 2 vs Player 3

This is the minimum number of days required.

Sample Input 2

4
2 3 4
1 3 4
4 1 2
3 1 2

Sample Output 2

4

All the conditions can be satisfied if the matches are scheduled for four days as follows:

Day 1: Player 1 vs Player 2, Player 3 vs Player 4

Day 2: Player 1 vs Player 3

Day 3: Player 1 vs Player 4, Player 2 vs Player 3

Day 4: Player 2 vs Player 4

This is the minimum number of days required.

Sample Input 3

3
2 3
3 1
1 2

Sample Output 3

-1

Any scheduling of the matches violates some condition.

Solution in Ruby (2.3.3) (judge verdict: Accepted)

# coding: utf-8
N = gets().to_i
btls = []
comp = [true] + Array.new(N, false)
c_mem = (1..N).to_a
N.times do |i|
  btls << gets().chomp!.split(" ").map{|e| e.to_i}
end
btls.unshift([])

ans = 0
# 対戦が全部完了するまで繰り返し
while !comp.all?
  ans += 1
  next_mem = []
  matched = Array.new(N+1, false)
  
  c_mem.each do |i|
    # すでに全戦終了済みだったらスキップ
    next if comp[i] or comp[btls[i][0]]
    vs = btls[i][0]
    
    # 相手の対戦順序と同じかどうか？
    if !matched[i] && !matched[vs] && i == btls[vs][0] 
      btls[i].shift
      btls[vs].shift
      next_mem << i << vs
      matched[i] = true
      matched[vs] = true
      comp[i] = true if btls[i].empty?
      comp[vs] = true if btls[vs].empty?
    end
  end
  if next_mem.empty?
    puts -1
    exit 0
  end
  c_mem = next_mem
end

puts ans