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.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.Writer;
import java.lang.reflect.Constructor;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.InputMismatchException;
import java.util.StringTokenizer;

public class CodeB {
  private static final boolean IN_DEBUG = true;

  static final SuperWriter sw = new SuperWriter(System.out);

  static boolean hasThree(boolean[][] board) {
    for (int i = 0; i < board.length; i++) {
      for (int j = 0; j < board.length; j++) {
        int count = 0;
        for (int k = 0; k < 3; k++) {
          for (int l = 0; l < 3; l++) {
            if (i + k < board.length && j + l < board.length) {
              if (board[i + k][j + l]) {
                count++;
              }
            }
          }
        }
        if (count == 9) {
          return true;
        }
      }
    }
    return false;
  }
  
  static int n;
  static int[][] moves;

  static boolean simulateUpTo(int to) {
    boolean[][] matrix = new boolean[n][n];
    for (int i = 0; i <= to; i++) {
      matrix[moves[i][0]][moves[i][1]] = true;
    }
    return hasThree(matrix);
  }

  static int binarySearch(int from, int to) {
    if (from == to) {
      return simulateUpTo(from) ? from + 1 : -1;
    }
    if (from + 1 == to) {
      return simulateUpTo(from) ? from + 1 : simulateUpTo(to) ? to + 1 : -1;
    }
    int mid = (from + to) / 2;
    if (simulateUpTo(mid)) {
      return binarySearch(from, mid);
    } else {
      return binarySearch(mid + 1, to);
    }
  }

  public static void main(String[] args) {
    Scanner sc = new SuperScanner();
    n = sc.nextIntOrQuit();
    int m = sc.nextInt();
    moves = new int[m][2];
    for (int i = 0; i < m; i++) {
      moves[i][0] = sc.nextInt() - 1;
      moves[i][1] = sc.nextInt() - 1;
    }
    sw.printLine(binarySearch(0, m - 1));
  }

  static class SuperScanner extends Scanner {
    private InputStream stream;
    private byte[] buf = new byte[8096];
    private int curChar;
    private int numChars;

    public SuperScanner() {
        this.stream = System.in;
    }

    public int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }
    
    public static boolean isWhitespace(int c) {
      return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }
    
    private final StringBuilder sb = new StringBuilder();
    
    @Override
    public String next() {
      int c = read();
      while (isWhitespace(c)) {
          if (c == -1) {
            return null;
          }
          c = read();
      }
      sb.setLength(0);
      do {
          sb.append((char) c);
          c = read();
      } while (!isWhitespace(c));
      return sb.toString();
    }

    @Override
    public int nextInt() {
        int c = read();
        while (isWhitespace(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res = (res << 3) + (res << 1);
            res += c - '0';
            c = read();
        } while (!isWhitespace(c));
        return res * sgn;
    }

    @Override
    public long nextLong() {
        int c = read();
        while (isWhitespace(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        long res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res = (res << 3) + (res << 1);
            res += c - '0';
            c = read();
        } while (!isWhitespace(c));
        return res * sgn;
    }
  }
  
  static class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");

    public String nextLine() {
      try {
        return br.readLine();
      } catch (Exception e) {
        throw (new RuntimeException());
      }
    }

    public int nextIntOrQuit() {
      Integer n = nextInteger();
      if (n == null) {
        sw.close();
        System.exit(0);
      }
      return n.intValue();
    }

    public String next() {
      while (!st.hasMoreTokens()) {
        String l = nextLine();
        if (l == null)
          return null;
        st = new StringTokenizer(l);
      }
      return st.nextToken();
    }

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

    public long nextLong() {
      return Long.parseLong(next());
    }

    public double nextDouble() {
      return Double.parseDouble(next());
    }

    public int[] nextIntArray(int n) {
      int[] res = new int[n];
      for (int i = 0; i < res.length; i++)
        res[i] = nextInt();
      return res;
    }

    public long[] nextLongArray(int n) {
      long[] res = new long[n];
      for (int i = 0; i < res.length; i++)
        res[i] = nextLong();
      return res;
    }

    public double[] nextDoubleArray(int n) {
      double[] res = new double[n];
      for (int i = 0; i < res.length; i++)
        res[i] = nextDouble();
      return res;
    }

    public void sortIntArray(int[] array) {
      Integer[] vals = new Integer[array.length];
      for (int i = 0; i < array.length; i++)
        vals[i] = array[i];
      Arrays.sort(vals);
      for (int i = 0; i < array.length; i++)
        array[i] = vals[i];
    }

    public void sortLongArray(long[] array) {
      Long[] vals = new Long[array.length];
      for (int i = 0; i < array.length; i++)
        vals[i] = array[i];
      Arrays.sort(vals);
      for (int i = 0; i < array.length; i++)
        array[i] = vals[i];
    }

    public void sortDoubleArray(double[] array) {
      Double[] vals = new Double[array.length];
      for (int i = 0; i < array.length; i++)
        vals[i] = array[i];
      Arrays.sort(vals);
      for (int i = 0; i < array.length; i++)
        array[i] = vals[i];
    }

    public String[] nextStringArray(int n) {
      String[] vals = new String[n];
      for (int i = 0; i < n; i++)
        vals[i] = next();
      return vals;
    }

    public Integer nextInteger() {
      String s = next();
      if (s == null)
        return null;
      return Integer.parseInt(s);
    }

    public int[][] nextIntMatrix(int n, int m) {
      int[][] ans = new int[n][];
      for (int i = 0; i < n; i++)
        ans[i] = nextIntArray(m);
      return ans;
    }

    public char[][] nextGrid(int r) {
      char[][] grid = new char[r][];
      for (int i = 0; i < r; i++)
        grid[i] = next().toCharArray();
      return grid;
    }

    public static <T> T fill(T arreglo, int val) {
      if (arreglo instanceof Object[]) {
        Object[] a = (Object[]) arreglo;
        for (Object x : a)
          fill(x, val);
      } else if (arreglo instanceof int[])
        Arrays.fill((int[]) arreglo, val);
      else if (arreglo instanceof double[])
        Arrays.fill((double[]) arreglo, val);
      else if (arreglo instanceof long[])
        Arrays.fill((long[]) arreglo, val);
      return arreglo;
    }

    public <T> T[] nextObjectArray(Class<T> clazz, int size) {
      @SuppressWarnings("unchecked")
      T[] result = (T[]) java.lang.reflect.Array.newInstance(clazz, size);
      for (int c = 0; c < 3; c++) {
        Constructor<T> constructor;
        try {
          if (c == 0)
            constructor = clazz.getDeclaredConstructor(Scanner.class,
                Integer.TYPE);
          else if (c == 1)
            constructor = clazz.getDeclaredConstructor(Scanner.class);
          else
            constructor = clazz.getDeclaredConstructor();
        } catch (Exception e) {
          continue;
        }
        try {
          for (int i = 0; i < result.length; i++) {
            if (c == 0)
              result[i] = constructor.newInstance(this, i);
            else if (c == 1)
              result[i] = constructor.newInstance(this);
            else
              result[i] = constructor.newInstance();
          }
        } catch (Exception e) {
          throw new RuntimeException(e);
        }
        return result;
      }
      throw new RuntimeException("Constructor not found");
    }

    public Collection<Integer> wrap(int[] as) {
      ArrayList<Integer> ans = new ArrayList<Integer>();
      for (int i : as)
        ans.add(i);
      return ans;
    }

    public int[] unwrap(Collection<Integer> collection) {
      int[] vals = new int[collection.size()];
      int index = 0;
      for (int i : collection)
        vals[index++] = i;
      return vals;
    }
  }

  static class SuperWriter {

    private final PrintWriter writer;

    public SuperWriter(OutputStream outputStream) {
      writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
    }

    public SuperWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }

    public void close() {
        writer.flush();
        writer.close();
    }

    public void printLine(String line) {
      writer.println(line);
      if (IN_DEBUG) {
        writer.flush();
      }
    }

    public void printLine(int... vals) {
      if (vals.length == 0) {
        writer.println();
      } else {
        writer.print(vals[0]);
        for (int i = 1; i < vals.length; i++)
          writer.print(" ".concat(String.valueOf(vals[i])));
        writer.println();
      }
      if (IN_DEBUG) {
        writer.flush();
      }
    }

    public void printLine(long... vals) {
      if (vals.length == 0) {
        writer.println();
      } else {
        writer.print(vals[0]);
        for (int i = 1; i < vals.length; i++)
          writer.print(" ".concat(String.valueOf(vals[i])));
        writer.println();
      }
      if (IN_DEBUG) {
        writer.flush();
      }
    }

    public void printLine(double... vals) {
      if (vals.length == 0) {
        writer.println();
      } else {
        writer.print(vals[0]);
        for (int i = 1; i < vals.length; i++)
          writer.print(" ".concat(String.valueOf(vals[i])));
        writer.println();
      }
      if (IN_DEBUG) {
        writer.flush();
      }
    }

    public void printLine(int prec, double... vals) {
      if (vals.length == 0) {
        writer.println();
      } else {
        String precision = "%." + prec + "f";
        writer.print(String.format(precision, vals[0]).replace(',', '.'));
        precision = " " + precision;
        for (int i = 1; i < vals.length; i++)
          writer.print(String.format(precision, vals[i]).replace(',', '.'));
        writer.println();
      }
      if (IN_DEBUG) {
        writer.flush();
      }
    }

    public <E> void printLine(Collection<E> vals) {
      if (vals.size() == 0) {
        writer.println();
      } else {
        int i = 0;
        for (E val : vals) {
          if (i++ == 0) {
            writer.print(val.toString());
          } else {
            writer.print(" ".concat(val.toString()));
          }
        }
        writer.println();
      }
      if (IN_DEBUG) {
        writer.flush();
      }
    }

    public void print(String value) {
      writer.print(value);
      if (IN_DEBUG) {
        writer.flush();
      }
    }
  }

}

