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 Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashSet;

public class Main {

	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		String[] inputs = br.readLine().split(" ");
		int N = Integer.parseInt(inputs[0]);

		int tb[][] = new int[N + 1][N]; // pのn番目の対戦相手 r = tb[p][n]  (1-origin 
		int pIdx[] = new int[N + 1];	// pの試合消化数 = pIdx[p]       同上

		// p:player  r:rival
		for ( int p = 1; p <= N; p++ ) {
			pIdx[p] = 1;

			inputs = br.readLine().split(" ");
			for ( int n = 1; n <= N - 1; n++ ) {
				int r = Integer.parseInt(inputs[n - 1]);
				tb[p][n] = r;
			}
		}

		int match = N * (N - 1) / 2;
		int days = 0;

		HashSet<Integer> todayDone = new HashSet<>(); // その日の試合が成立して除外された選手をここに入れる
		do {
			days++;
			todayDone.clear();

			for ( int p = 1; p <= N; p++ ) {
				if ( todayDone.contains(p) || pIdx[p] >= N ) continue;

				int r = tb[p][pIdx[p]];
				if ( todayDone.contains(r) || pIdx[p] >= N ) continue;
				
				// pの「相手の相手」がpだったら試合成立
				int r_r = tb[r][pIdx[r]];				
				if ( p == r_r ) {
					// todayDone.add(p);
					todayDone.add(r);
					pIdx[p]++;
					pIdx[r]++;
					match--;
				}
				
				// pについて見終わっている
				todayDone.add(p);
			}

		} while ( todayDone.size() > 0 );

		System.out.println(match == 0 ? days - 1 : -1);
		br.close();
	}
}