One not particularly beautiful evening Valera got very bored. To amuse himself a little bit, he found the following game.

He took a checkered white square piece of paper, consisting of n × n cells. After that, he started to paint the white cells black one after the other. In total he painted m different cells on the piece of paper. Since Valera was keen on everything square, he wondered, how many moves (i.e. times the boy paints a square black) he should make till a black square with side 3 can be found on the piece of paper. But Valera does not know the answer to this question, so he asks you to help him.

Your task is to find the minimum number of moves, till the checkered piece of paper has at least one black square with side of 3. Otherwise determine that such move does not exist.

Input

The first line contains two integers n and m (1 ≤ n ≤ 1000, 1 ≤ m ≤ min(n·n, 105)) — the size of the squared piece of paper and the number of moves, correspondingly. 

Then, m lines contain the description of the moves. The i-th line contains two integers xi, yi (1 ≤ xi, yi ≤ n) — the number of row and column of the square that gets painted on the i-th move. 

All numbers on the lines are separated by single spaces. It is guaranteed that all moves are different. The moves are numbered starting from 1 in the order, in which they are given in the input. The columns of the squared piece of paper are numbered starting from 1, from the left to the right. The rows of the squared piece of paper are numbered starting from 1, from top to bottom.

Output

On a single line print the answer to the problem — the minimum number of the move after which the piece of paper has a black square with side 3. If no such move exists, print -1.

Examples

Input

4 11
1 1
1 2
1 3
2 2
2 3
1 4
2 4
3 4
3 2
3 3
4 1


Output

10


Input

4 12
1 1
1 2
1 3
2 2
2 3
1 4
2 4
3 4
3 2
4 2
4 1
3 1


Output

-1
/*
ID: sunits789
LANG: JAVA
TASK: B128
*/
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.Arrays;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.math.BigInteger;
import java.io.InputStream;
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.FileReader;


public class B128 {
        public static void main(String[] args) {
                // String inputStream = "file.in";
                InputStream inputStream = System.in;
                // String outputStream = "file.out";
                OutputStream outputStream = System.out;
                InputReader in = new InputReader(inputStream);
                PrintWriter out = new PrintWriter(outputStream);
				// PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter(oFile)));
                CodeB128 solver = new CodeB128();
                solver.solve(1, in, out);
                out.close();
        }
}

class CodeB128
{
	public void solve(int testNumber, InputReader in, PrintWriter out)
	{
		int n=in.nextInt();
		int m=in.nextInt();
		boolean board[][]=new boolean[n][n];
		for(int i=0;i<n;i++)
		{
			Arrays.fill(board[i],false);
		}
		int ans=-1;
		for(int i=0;i<m;i++)
		{
			int r=in.nextInt()-1;
			int c=in.nextInt()-1;
			board[r][c]=true;
			if(check(board,n,r,c))
			{
				ans=i+1;
				// out.println(r+" "+c);
				// for(int j=0;j<n;j++)
				// {
					// for(int k=0;k<n;k++)
					// {
						// out.print(board[j][k]);
					// }
					// out.println();
				// }
				i=m;
			}
		}
		out.println(ans);
	}
	
	public boolean check(boolean board[][],int n,int r,int c)
	{
		boolean ret=false;
		for(int srr=-2;srr<=0;srr++)
		{
			for(int scc=-2;scc<=0;scc++)
			{
				int sr=r+srr;
				int sc=c+scc;
				if(sr<0||sc<0||(sr+2)>=n||(sc+2)>=n)
				{
					continue;
				}
				else
				{
					boolean temp=board[sr][sc];
					for(int i=0;i<3;i++)
					{
						for(int j=0;j<3;j++)
						{
							int tr=sr+i;
							int tc=sc+j;
							temp=temp&&board[tr][tc];
						}
					}
					if(temp)
					{
						return temp;
					}
				}
			}
		}
		return ret;
	}
}






class InputReader {
    private BufferedReader reader;
    private StringTokenizer tokenizer;

    public InputReader(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream));
        tokenizer = null;
    }
	
	public InputReader(String iFile)throws IOException {
        reader = new BufferedReader(new FileReader(iFile));
        tokenizer = null;
    }

    public String next() {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public long nextLong() {
        return Long.parseLong(next());
    }
	public void inputShuffleArrayInt(int arr[])
	{
		int l=arr.length;
		for(int i=0;i<l;i++)
		{
			if(i%2==0)
			{
				arr[i/2]=nextInt();
			}
			else
			{
				arr[l-((i+1)/2)]=nextInt();
			}
		}
	}

    }