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.*;

public class Template implements Runnable {

    BufferedReader in;
    PrintWriter out;
    StringTokenizer tok = new StringTokenizer("");

    void init() throws FileNotFoundException {
        try {
            in = new BufferedReader(new FileReader("input.txt"));
            out = new PrintWriter("output.txt");
        } catch (Exception e) {
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);
        }
    }

    class GraphBuilder {
        int n, m;
        int[] x, y;
        int index;
        int[] size;


        GraphBuilder(int n, int m) {
            this.n = n;
            this.m = m;
            x = new int[m];
            y = new int[m];
            size = new int[n];
        }

        void add(int u, int v) {
            x[index] = u;
            y[index] = v;
            size[u]++;
            size[v]++;
            index++;
        }

        int[][] build() {
            int[][] graph = new int[n][];
            for (int i = 0; i < n; i++) {
                graph[i] = new int[size[i]];
            }
            for (int i = index - 1; i >= 0; i--) {
                int u = x[i];
                int v = y[i];
                graph[u][--size[u]] = v;
                graph[v][--size[v]] = u;
            }
            return graph;
        }
    }

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

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

    int[] readIntArray(int size) throws IOException {
        int[] res = new int[size];
        for (int i = 0; i < size; i++) {
            res[i] = readInt();
        }
        return res;
    }

    long[] readLongArray(int size) throws IOException {
        long[] res = new long[size];
        for (int i = 0; i < size; i++) {
            res[i] = readLong();
        }
        return res;
    }

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

    double readDouble() throws IOException {
        return Double.parseDouble(readString());
    }

    <T> List<T>[] createGraphList(int size) {
        List<T>[] list = new List[size];
        for (int i = 0; i < size; i++) {
            list[i] = new ArrayList<>();
        }
        return list;
    }

    public static void main(String[] args) {
        new Template().run();
        // new Thread(null, new Template(), "", 1l * 200 * 1024 * 1024).start();
    }

    long timeBegin, timeEnd;

    void time() {
        timeEnd = System.currentTimeMillis();
        System.err.println("Time = " + (timeEnd - timeBegin));
    }

    long memoryTotal, memoryFree;

    void memory() {
        memoryFree = Runtime.getRuntime().freeMemory();
        System.err.println("Memory = " + ((memoryTotal - memoryFree) >> 10)
                + " KB");
    }

    public void run() {
        try {
            timeBegin = System.currentTimeMillis();
            memoryTotal = Runtime.getRuntime().freeMemory();
            init();
            solve();
            out.close();
            if (System.getProperty("ONLINE_JUDGE") == null) {
                time();
                memory();
            }
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }

    class Cup implements Comparable<Cup> {
        int a;
        int index;

        public Cup(int a, int index) {
            this.a = a;
            this.index = index;
        }

        @Override
        public int compareTo(Cup o) {
            return o.a - a;
        }
    }

    void add(Map<Long, Integer> map, long v, int d) {
        map.put(v, map.getOrDefault(v, 0) + d);
        if (map.getOrDefault(v, 0) == 0) {
            map.remove(v);
        }
    }

    boolean ok(int si, int sj, int[][] data, int n) {
        if (si + 2 >= n) return false;
        if (sj + 2 >= n) return false;
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                if (data[si + i][sj + j] == 0) return false;
            }
        }
        return true;
    }

    void solve() throws IOException {
        int n = readInt();
        int m = readInt();
        int[][] data = new int[n][n];
        for (int step = 1; step <= m; step++) {
            int i = readInt() - 1;
            int j = readInt() - 1;

            data[i][j] = 1;
            int fromI = Math.max(0, i - 2);
            int fromJ = Math.max(0, j - 2);
            for (int si = fromI; si <= i; si++) {
                for (int sj = fromJ; sj <= j; sj++) {
                    if (ok(si, sj, data, n)) {
                        out.println(step);
                        return;
                    }
                }
            }

        }
        out.println(-1);
    }


}