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

public class B implements Runnable {

	final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
	
	BufferedReader in;
	PrintWriter out;
	StringTokenizer tok;

	public static void main(String[] args) {
		new Thread(null, new B(), "", 256 * (1L << 20)).start();
	}

	@Override
	public void run() {
		try {
			long startTime = System.currentTimeMillis();
			if (ONLINE_JUDGE) {
				in = new BufferedReader(new InputStreamReader(System.in));
				out = new PrintWriter(System.out);
			} else {
				in = new BufferedReader(new FileReader("input.txt"));
				out = new PrintWriter("output.txt");
			}
			Locale.setDefault(Locale.US);
			tok = new StringTokenizer("");
			solve();
			in.close();
			out.close();
			long freeMemory = Runtime.getRuntime().freeMemory();
			long totalMemory = Runtime.getRuntime().totalMemory();
			long endTime = System.currentTimeMillis();
			System.err.println("Time = " + (endTime - startTime));
			System.err.println("Memory = " + ((totalMemory - freeMemory) >> 10));
		} catch (Throwable t) {
			t.printStackTrace(System.err);
			System.exit(-1);
		}
	}

	String readString() throws IOException {
		while (!tok.hasMoreTokens()) {
			tok = new StringTokenizer(in.readLine());
		}
		return tok.nextToken();
	}

	int readInt() throws IOException {
		return Integer.parseInt(readString());
	}

	long readLong() throws IOException {
		return Long.parseLong(readString());
	}

	double readDouble() throws IOException {
		return Double.parseDouble(readString());
	}
	
	void debug(Object... o) {
		if (!ONLINE_JUDGE) {
			System.err.println(Arrays.deepToString(o));
		}
	}

	static class Utils {

		private Utils() {}

		public static void mergeSort(int[] a) {
			mergeSort(a, 0, a.length - 1);
		}

		private static final int MAGIC_VALUE = 50;

		private static void mergeSort(int[] a, int leftIndex, int rightIndex) {
			if (leftIndex < rightIndex) {
				if (rightIndex - leftIndex <= MAGIC_VALUE) {
					insertionSort(a, leftIndex, rightIndex);
				} else {
					int middleIndex = (leftIndex + rightIndex) / 2;
					mergeSort(a, leftIndex, middleIndex);
					mergeSort(a, middleIndex + 1, rightIndex);
					merge(a, leftIndex, middleIndex, rightIndex);
				}
			}
		}

		private static void merge(int[] a, int leftIndex, int middleIndex, int rightIndex) {
			int length1 = middleIndex - leftIndex + 1;
			int length2 = rightIndex - middleIndex;
			int[] leftArray = new int[length1];
			int[] rightArray = new int[length2];
			System.arraycopy(a, leftIndex, leftArray, 0, length1);
			System.arraycopy(a, middleIndex + 1, rightArray, 0, length2);
			for (int k = leftIndex, i = 0, j = 0; k <= rightIndex; k++) {
				if (i == length1) {
					a[k] = rightArray[j++];
				} else if (j == length2) {
					a[k] = leftArray[i++];
				} else {
					a[k] = leftArray[i] <= rightArray[j] ? leftArray[i++] : rightArray[j++];
				}
			}
		}

		private static void insertionSort(int[] a, int leftIndex, int rightIndex) {
			for (int i = leftIndex + 1; i <= rightIndex; i++) {
				int current = a[i];
				int j = i - 1;
				while (j >= leftIndex && a[j] > current) {
					a[j + 1] = a[j];
					j--;
				}
				a[j + 1] = current;
			}
		}

	}

	// solution
	
	void solve() throws IOException {
		int n = readInt();
		int m = readInt();
		boolean[][] a = new boolean[n][n];
		for (int i = 0; i < m; i++) {
			int x = readInt() - 1;
			int y = readInt() - 1;
			a[x][y] = true;
			for (int dx = -1; dx <= 1; dx++) {
				for (int dy = -1; dy <= 1; dy++) {
					if (good(x + dx, y + dy, n) && ok(a, x + dx, y + dy, n)) {
						out.println(i + 1);
						return;
					}	
				}
			}
			
		}
		out.println(-1);
	}

	private boolean ok(boolean[][] a, int x, int y, int n) {
		for (int dx = -1; dx <= 1; dx++) {
			for (int dy = -1; dy <= 1; dy++) {
				if (!good(x + dx, y + dy, n) || !a[x + dx][y + dy]) {
					return false;
				}
			}
		}
		return true;
	}

	private boolean good(int x, int y, int n) {
		return x >= 0 && x < n && y >= 0 && y < n;
	}

}
