Problem:
Score : 600 points

Problem Statement

We have 3N cards arranged in a row from left to right, where each card has an integer between 1 and N (inclusive) written on it. The integer written on the i-th card from the left is A_i.

You will do the following operation N-1 times:

Rearrange the five leftmost cards in any order you like, then remove the three leftmost cards. If the integers written on those three cards are all equal, you gain 1 point.

After these N-1 operations, if the integers written on the remaining three cards are all equal, you will gain 1 additional point.

Find the maximum number of points you can gain.

Constraints

1 \leq N \leq 2000

1 \leq A_i \leq N

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 \cdots A_{3N}

Output

Print the maximum number of points you can gain.

Sample Input 1

2
1 2 1 2 2 1

Sample Output 1

2

Let us rearrange the five leftmost cards so that the integers written on the six cards will be 2\ 2\ 2\ 1\ 1\ 1 from left to right.

Then, remove the three leftmost cards, all of which have the same integer 2, gaining 1 point.

Now, the integers written on the remaining cards are 1\ 1\ 1.

Since these three cards have the same integer 1, we gain 1 more point.

In this way, we can gain 2 points - which is the maximum possible.

Sample Input 2

3
1 1 2 2 3 3 3 2 1

Sample Output 2

1

Sample Input 3

3
1 1 2 2 2 3 3 3 1

Sample Output 3

3

Solution in C# (Mono-mcs 6.8.0.105) (judge verdict: Wrong Answer)


using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
using System.Net.WebSockets;

class Test
{

    // Driver code 
    public static void Main()
    {
        ///var t = int.Parse(Console.ReadLine());

        //while (t > 0)
        {
            //  t--;

            var ii = Console.ReadLine().Split(' ').Where(xx => !string.IsNullOrEmpty(xx)).Select(xx => int.Parse(xx)).ToArray();

            var n = ii[0];
            // var x = ii[1];
            // var tt = ii[2];

            var arr = Console.ReadLine().Split(' ').Where(x => !string.IsNullOrEmpty(x)).Select(xx => int.Parse(xx)).ToList();
            int ans = 0;
            for (int i = 0; i < arr.Count; i++)
            {
                int[] fre = new int[n + 1];
                bool check = false;
                for (int j = 0; j < Math.Min(5,arr.Count); j++)
                {
                    fre[arr[j]]++;
                    if (fre[arr[j]] == 3)
                    {
                        check = true;
                        ans++;
                        int x = 0;
                        var el = arr[j];
                        while (x < 3)
                        {
                            //if (arr[k] == el && x < 3)
                            {
                                arr.Remove(el);
                                x++;
                            }
                        }
                        break;
                    }
                    
                }
                if(!check)
                {
                    arr.RemoveRange(0, 3);
                }
            }

            Console.Write(ans);
            Console.WriteLine();
        }

       
    }
}