package graph;

import java.util.LinkedList;

import java.util.Queue;

import java.util.Scanner;

public class tomato {

public static int n, m, day;

public static int box[][];

static int[] dx = {0,1,0,-1};

static int[] dy = {1,0,-1,0};

static class Toma{

int x;

int y;

int day;

public Toma(int x, int y, int day) {

this.x = x;

this.y = y;

this.day = day;

}

}

public static void bfs() {

Queue<Toma> q = new LinkedList<Toma>();

day = 0;

for(int i=0; i<m; i++) {

for(int j=0; j<n; j++) {

if(box[i][j]==1)

q.offer((new Toma(i,j,0)));

}

}

while(!q.isEmpty()) {

Toma toma = q.poll();

day = toma.day;

for(int i=0; i<4; i++) {

int nx = toma.x + dx[i];

int ny = toma.y + dy[i];

if(nx >= 0 && ny >=0 && nx < m && ny < n) {

if(box[nx][ny]==0) {

box[nx][ny]=1;

q.add(new Toma(nx,ny,day+1));

}

}

}

}

}

public static void main(String[] args) {

//#7576¹ø_Å丶Åä

Scanner sc = new Scanner(System.in);

n = sc.nextInt();

m = sc.nextInt();

box = new int[1000][1000];

for(int i =0; i<m; i++) {

for(int j=0; j<n; j++) {

box[i][j] = sc.nextInt();

}

}

bfs();

if(checkTomato())

System.out.println(day);

else

System.out.println(-1);

}

static boolean checkTomato() {

for(int i=0; i<m; i++) {

for(int j=0; j<n; j++) {

if(box[i][j] == 0)

return false;

}

}

return true;

}

}