import java.io.*;
import java.util.*;

public class Main {
	static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	static StringTokenizer st;
	static int ROW, COLUMN, total;
	static int days = -1;
	static int[][] board;
	static boolean[][] isVisited;
	static int[] dr = {0,0,1,-1};
	static int[] dc = {1,-1,0,0};
	// Queue에서 좌표로 사용하기 위한 class
	private static class Pos {
	   int r, c;

	   Pos(int r, int c) {
	      this.r = r;
	      this.c = c;
	   }
	}

	public static void main(String[] args) throws IOException {
		// 입력
		st = new StringTokenizer(br.readLine());
		COLUMN = Integer.parseInt(st.nextToken());
		ROW = Integer.parseInt(st.nextToken());
		board = new int[ROW][COLUMN];
		isVisited = new boolean[ROW][COLUMN];
		total = ROW * COLUMN;
		Queue<Pos> queue = new LinkedList<>();

		for(int r=0; r < ROW ; r++) {
			st = new StringTokenizer(br.readLine());
			for(int c=0; c < COLUMN; c++) {
				int tomato = Integer.parseInt(st.nextToken());
				switch (tomato) {
					case 1: {
						queue.add(new Pos(r,c));
						total -= tomato;
						break;
					}
					case -1: {
						total += tomato;
						break;
					}
					default: {
						total += tomato;
					}
				}
				board[r][c] = tomato;
			}
		}
		//BFS
		while(queue.size() > 0) {
			int nowCount = queue.size();
			days++;

			while(nowCount-- > 0) {
				Pos now = queue.poll();

				for(int i=0; i<4; i++) {
					int nextR = now.r + dr[i];
					int nextC = now.c + dc[i];
					boolean isRange = nextR < 0 || nextR >=ROW || nextC < 0 || nextC >= COLUMN;

					if(isRange || isVisited[nextR][nextC]) continue;

					isVisited[nextR][nextC] = true;

					if(board[nextR][nextC] == 0) {
						queue.add(new Pos(nextR, nextC));
						isVisited[nextR][nextC] = true;
						total--;
					}
				}
			}
		}

		if(total > 0) {
			System.out.println(-1);
		} else {
			System.out.println(days);
		}
	}
}