using System;
using System.Collections.Generic;
using System.Linq;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;
using TextReader = System.IO.TextReader;
using System.Text;

namespace AtCoderProject
{
    public class Program
    {
        public object Calc()
        {
            var N = consoleReader.Int;
            var a = consoleReader.Repeat(N).Select(cr => cr.Split.Int0).ToArray();
            var matchNum = N * (N - 1) / 2;
            var gb = new GraphBuilder(matchNum);
            for (int i = 0; i < N; i++)
            {
                int prevMatch, currentMatch;
                prevMatch = MatchNumber(i, a[i][0], N);
                for (int j = 1; j < N - 1; j++)
                {
                    currentMatch = MatchNumber(i, a[i][j], N);
                    gb.Add(prevMatch, currentMatch);
                    prevMatch = currentMatch;
                }
            }

            var days = 0;
            var current = new HashSet<Node>();
            var match = 0;

            foreach (var node in gb.nodes)
                if (node.roots.Count == 0)
                    current.Add(node);

            while (match < matchNum)
            {
                if (current.Count == 0) return -1;

                ++days;
                var arr = current.ToArray();
                current.Clear();
                foreach (var node in arr)
                {
                    ++match;
                    foreach (var child in node.children)
                    {
                        if (child.roots.Count > 1)
                            child.roots.Remove(node);
                        else
                            current.Add(child);
                    }
                }
            }
            return days;
        }
        static int MatchNumber(int a, int b, int N)
        {
            if (a == b) throw new Exception();
            else if (a > b) return MatchNumber(b, a, N);
            return a * N + b - (a + 1) * (a + 2) / 2;
        }

        class GraphBuilder
        {
            public Node[] nodes;
            public GraphBuilder(int count)
            {
                this.nodes = new Node[count];
                for (int i = 0; i < count; i++)
                    this.nodes[i] = new Node(i);
            }
            public GraphBuilder(int count, IEnumerable<int[]> arrays) : this(count)
            {
                foreach (var item in arrays)
                    this.Add(item[0], item[1]);
            }
            public void Add(int from, int to)
            {
                nodes[from].children.Add(nodes[to]);
                nodes[to].roots.Add(nodes[from]);
            }
        }
        public struct Node : IEquatable<Node>
        {
            public Node(int i)
            {
                this.index = i;
                this.roots = new HashSet<Node>();
                this.children = new HashSet<Node>();
            }
            public int index;
            public HashSet<Node> roots;
            public HashSet<Node> children;

            public override bool Equals(object obj)
            {
                if (obj is Node)
                    return this.Equals((Node)obj);
                else
                    return false;
            }
            public bool Equals(Node other) => this.index == other.index;
            public override int GetHashCode() => this.index;
        }

        #region いつもの
        private readonly ConsoleReader consoleReader;
        public Program(ConsoleReader consoleReader) { this.consoleReader = consoleReader; }
        static void Main() => Console.WriteLine(new Program(new ConsoleReader(Console.In)).Calc()); static string AllLines<T>(IEnumerable<T> source) => string.Join("\n", source);
        #endregion
    }
#pragma warning disable
    public class ConsoleReader
    {
        private string[] ReadLineSplit() => textReader.ReadLine().Split(Array.Empty<char>(), StringSplitOptions.RemoveEmptyEntries);
        private string[] line = Array.Empty<string>();
        private int linePosition;
        private TextReader textReader;
        public ConsoleReader(TextReader tr) { textReader = tr; }
        public int Int => int.Parse(String);
        public long Long => long.Parse(String);
        public double Double => double.Parse(String);
        public string String
        {
            get
            {
                if (linePosition >= line.Length)
                {
                    linePosition = 0;
                    line = ReadLineSplit();
                }
                return line[linePosition++];
            }
        }
        public class SplitLine
        {
            private string[] splited;
            public SplitLine(ConsoleReader cr) { splited = cr.ReadLineSplit(); cr.line = Array.Empty<string>(); }
            public int[] Int => String.Select(x => int.Parse(x)).ToArray();
            public int[] Int0 => String.Select(x => int.Parse(x) - 1).ToArray();
            public long[] Long => String.Select(x => long.Parse(x)).ToArray();
            public double[] Double => String.Select(x => double.Parse(x)).ToArray();
            public string[] String => splited;
        }
        public SplitLine Split => new SplitLine(this);
        public class RepeatReader : IEnumerable<ConsoleReader>
        {
            ConsoleReader cr; int count;
            public RepeatReader(ConsoleReader cr, int count) { this.cr = cr; this.count = count; }
            public IEnumerator<ConsoleReader> GetEnumerator() => Enumerable.Repeat(cr, count).GetEnumerator();
            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator();

            public IEnumerable<string> String => this.Select(cr => cr.String);
            public IEnumerable<int> Int => this.Select(cr => cr.Int);
            public IEnumerable<int> Int0 => this.Select(cr => cr.Int - 1);
            public IEnumerable<long> Long => this.Select(cr => cr.Long);
            public IEnumerable<double> Double => this.Select(cr => cr.Double);
        }
        public RepeatReader Repeat(int count) => new RepeatReader(this, count);
    }
}

Measured memory_bytes: 316724