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
import static java.lang.Math.*;
import java.util.*;
import java.io.*;

public class B {

	public void solve() throws Exception {
		
		int n = nextInt(), m = nextInt();
//		startTimer();
		boolean[][] map = new boolean[n][n];
		for (int i=0; i<m; ++i) {
			int x = nextInt()-1, y = nextInt()-1;
			map[x][y] = true;
			for (int j=x-2; j<=x; ++j) if (j>=0 && j+2<n) for (int k=y-2; k<=y; ++k) if (k>=0 && k+2<n) {
				boolean ok = true;
				loop: for (int ii=j; ii<j+3; ++ii) for (int jj=k; jj<k+3; ++jj) {
					ok &= map[ii][jj];
					if (!ok) break loop;
				}
				if (ok) halt(i+1);
			}
		}
//		stopTimer();
		println(-1);
	}

	

	////////////////////////////////////////////////////////////////////////////
	
	boolean showDebug = true;
	static boolean useFiles = false;
	static String inFile = "input.txt";
	static String outFile = "output.txt";

	double EPS = 1e-7;
	int INF = Integer.MAX_VALUE;
	long INFL = Long.MAX_VALUE;
	double INFD = Double.MAX_VALUE;

	
	int absPos(int num) {
		return num<0 ? 0:num;
	}
	long absPos(long num) {
		return num<0 ? 0:num;
	}
	double absPos(double num) {
		return num<0 ? 0:num;
	}
	
	int min(int... nums) {
		int r = nums[0];
		for (int i=1; i<nums.length; ++i)
			if (nums[i]<r) r=nums[i];
		return r;
	}
	int max(int... nums) {
		int r = nums[0];
		for (int i=1; i<nums.length; ++i)
			if (nums[i]>r) r=nums[i];
		return r;
	}
	long minL(long... nums) {
		long r = nums[0];
		for (int i=1; i<nums.length; ++i)
			if (nums[i]<r) r=nums[i];
		return r;
	}
	long maxL(long... nums) {
		long r = nums[0];
		for (int i=1; i<nums.length; ++i)
			if (nums[i]>r) r=nums[i];
		return r;
	}
	double minD(double... nums) {
		double r = nums[0];
		for (int i=1; i<nums.length; ++i)
			if (nums[i]<r) r=nums[i];
		return r;
	}
	double maxD(double... nums) {
		double r = nums[0];
		for (int i=1; i<nums.length; ++i)
			if (nums[i]>r) r=nums[i];
		return r;
	}
	
	long sumArr(int[] arr) {
		long res = 0;
		for (int i=0; i<arr.length; ++i)
			res+=arr[i];
		return res;
	}
	long sumArr(long[] arr) {
		long res = 0;
		for (int i=0; i<arr.length; ++i)
			res+=arr[i];
		return res;
	}
	double sumArr(double[] arr) {
		double res = 0;
		for (int i=0; i<arr.length; ++i)
			res+=arr[i];
		return res;
	}

	long partsFitCnt(long partSize, long wholeSize) {
		return (partSize+wholeSize-1)/partSize;
	}

	boolean odd(long num) {
		return (num&1)==1;
	}
	
	boolean hasBit(int num, int pos) {
		return (num&(1<<pos))>0;
	}

	boolean isLetter(char c) {
		return (c>='a' && c<='z') || (c>='A' && c<='Z');
	}
	boolean isLowercase(char c) {
		return (c>='a' && c<='z');
	}
	boolean isUppercase(char c) {
		return (c>='A' && c<='Z');
	}
	boolean isDigit(char c) {
		return (c>='0' && c<='9');
	}
	
	boolean charIn(String chars, String s) {
		if (s==null) return false;
		if (chars==null || chars.equals("")) return true;
		for (int i=0; i<s.length(); ++i)
			for (int j=0; j<chars.length(); ++j)
				if (chars.charAt(j)==s.charAt(i)) return true;
		return false;
	}
	
	String stringn(String s, int n) {
		if (n<1 || s==null) return "";
		StringBuilder sb = new StringBuilder(s.length()*n);
		for (int i=0; i<n; ++i) sb.append(s);
		return sb.toString();
	}
	String str(Object o) {
		if (o==null) return "";
		return o.toString();
	}


	long timer = System.currentTimeMillis();
	void startTimer() {
		timer = System.currentTimeMillis();
	}
	void stopTimer() {
		System.err.println("time: "+(System.currentTimeMillis()-timer)/1000.0);
	}


	static class InputReader {
		private byte[] buf;
		private int bufPos = 0, bufLim = -1;
		private InputStream stream;
		public InputReader(InputStream stream, int size) {
			buf = new byte[size];
			this.stream = stream;
		}
		private void fillBuf() throws IOException {
			bufLim = stream.read(buf);
			bufPos = 0;
		}
		char read() throws IOException {
			if (bufPos>=bufLim) fillBuf(); 
			return (char)buf[bufPos++];
		}
		boolean hasInput() throws IOException {
			if (bufPos>=bufLim) fillBuf();
			return bufPos<bufLim;
		}
	}
	
	static InputReader inputReader;
	static BufferedWriter outputWriter;

	char nextChar() throws IOException {
		return inputReader.read();
	}
	char nextNonWhitespaceChar() throws IOException {
		char c = inputReader.read();
		while (c<=' ') c=inputReader.read();
		return c;
	}
	String nextWord() throws IOException {
		StringBuilder sb = new StringBuilder();
		char c = inputReader.read();
		while (c<=' ') c=inputReader.read();
		while (c>' ') {
			sb.append(c);
			c = inputReader.read();
		}
		return new String(sb);
	}
	String nextLine() throws IOException {
		StringBuilder sb = new StringBuilder();
		char c = inputReader.read();
		while (c<=' ') c=inputReader.read();
		while (c!='\n' && c!='\r') {
			sb.append(c);
			c = inputReader.read();
		}
		return new String(sb);
	}
	int nextInt() throws IOException {
		int r = 0;
		char c = nextNonWhitespaceChar();
		boolean neg = false;
		if (c=='-') neg=true;
		else r=c-48;
		c = nextChar();
		while (c>='0' && c<='9') {
			r*=10;
			r+=c-48;
			c=nextChar();
		}
		return neg ? -r:r;
	}
	long nextLong() throws IOException {
		long r = 0;
		char c = nextNonWhitespaceChar();
		boolean neg = false;
		if (c=='-') neg=true;
		else r = c-48;
		c = nextChar();
		while (c>='0' && c<='9') {
			r*=10L;
			r+=c-48L;
			c=nextChar();
		}
		return neg ? -r:r;
	}
	double nextDouble() throws NumberFormatException, IOException {
		return Double.parseDouble(nextWord());
	}
	int[] nextArr(int size) throws NumberFormatException, IOException {
		int[] arr = new int[size];
		for (int i=0; i<size; ++i)
			arr[i] = nextInt();
		return arr;
	}
	long[] nextArrL(int size) throws NumberFormatException, IOException {
		long[] arr = new long[size];
		for (int i=0; i<size; ++i)
			arr[i] = nextLong();
		return arr;
	}
	double[] nextArrD(int size) throws NumberFormatException, IOException {
		double[] arr = new double[size];
		for (int i=0; i<size; ++i)
			arr[i] = nextDouble();
		return arr;
	}
	String[] nextArrS(int size) throws NumberFormatException, IOException {
		String[] arr = new String[size];
		for (int i=0; i<size; ++i)
			arr[i] = nextWord();
		return arr;
	}
	char[] nextArrCh(int size) throws IOException {
		char[] arr = new char[size];
		for (int i=0; i<size; ++i)
			arr[i] = nextNonWhitespaceChar();
		return arr;
	}
	char[][] nextArrCh(int rows, int columns) throws IOException {
		char[][] arr = new char[rows][columns];
		for (int i=0; i<rows; ++i)
			for (int j=0; j<columns; ++j)
				arr[i][j] = nextNonWhitespaceChar();
		return arr;
	}
	char[][] nextArrChBorders(int rows, int columns, char border) throws IOException {
		char[][] arr = new char[rows+2][columns+2];
		for (int i=1; i<=rows; ++i)
			for (int j=1; j<=columns; ++j)
				arr[i][j] = nextNonWhitespaceChar();
		for (int i=0; i<columns+2; ++i) {
			arr[0][i] = border;
			arr[rows+1][i] = border;
		}
		for (int i=0; i<rows+2; ++i) {
			arr[i][0] = border;
			arr[i][columns+1] = border;
		}
		return arr;
	}

	void printf(String format, Object... args) throws IOException {
		outputWriter.write(String.format(format, args));
	}
	void print(Object o) throws IOException {
		outputWriter.write(o.toString());
	}
	void println(Object o) throws IOException {
		outputWriter.write(o.toString());
		outputWriter.newLine();
	}
	void print(Object... o) throws IOException {
		for (int i=0; i<o.length; ++i) {
			if (i!=0) outputWriter.write(' ');
			outputWriter.write(o[i].toString());
		}
	}
	void println(Object... o) throws IOException {
		print(o);
		outputWriter.newLine();
	}
	void printn(Object o, int n) throws IOException {
		String s = o.toString();
		for (int i=0; i<n; ++i) {
			outputWriter.write(s);
			if (i!=n-1) outputWriter.write(' ');
		}
	}
	void printnln(Object o, int n) throws IOException {
		printn(o, n);
		outputWriter.newLine();
	}
	void printArr(int[] arr) throws IOException {
		for (int i=0; i<arr.length; ++i) {
			if (i!=0) outputWriter.write(' ');
			outputWriter.write(Integer.toString(arr[i]));
		}
	}
	void printArr(long[] arr) throws IOException {
		for (int i=0; i<arr.length; ++i) {
			if (i!=0) outputWriter.write(' ');
			outputWriter.write(Long.toString(arr[i]));
		}
	}
	void printArr(double[] arr) throws IOException {
		for (int i=0; i<arr.length; ++i) {
			if (i!=0) outputWriter.write(' ');
			outputWriter.write(Double.toString(arr[i]));
		}
	}
	void printArr(String[] arr) throws IOException {
		for (int i=0; i<arr.length; ++i) {
			if (i!=0) outputWriter.write(' ');
			outputWriter.write(arr[i]);
		}
	}
	void printlnArr(int[] arr) throws IOException {
		printArr(arr);
		outputWriter.newLine();
	}
	void printlnArr(long[] arr) throws IOException {
		printArr(arr);
		outputWriter.newLine();
	}
	void printlnArr(double[] arr) throws IOException {
		printArr(arr);
		outputWriter.newLine();
	}
	void printlnArr(String[] arr) throws IOException {
		printArr(arr);
		outputWriter.newLine();
	}
	void halt(Object... o) throws IOException {
		if (o.length!=0) println(o);
		outputWriter.flush(); outputWriter.close();
		System.exit(0);
	}
	
	void debug(Object... o) {
		if (showDebug) System.err.println(Arrays.deepToString(o));
	}


	public static void main(String[] args) throws Exception {
		Locale.setDefault(Locale.US);
		if (!useFiles) {
			inputReader = new InputReader(System.in, 1<<16);
			outputWriter = new BufferedWriter(new OutputStreamWriter(System.out), 1<<16);
		} else {
			inputReader = new InputReader(new FileInputStream(new File(inFile)), 1<<16);
			outputWriter = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(new File(outFile))), 1<<16);
		}
		new B().solve();
		outputWriter.flush(); outputWriter.close();
	}
}
