#include <iostream>

#include <list>

#include <algorithm>

using namespace std;

int N, M;

int board[1001][1001];

bool visited[1001][1001];

int days = 1;

list<int> bfsQueueX;

list<int> bfsQueueY;

void bfs() {

if (bfsQueueX.empty()) return;

int x = bfsQueueX.front(); int y = bfsQueueY.front();

bfsQueueX.pop_front(); bfsQueueY.pop_front();

if (x > 0 && board[x - 1][y] == 0 && !visited[x - 1][y]) {

board[x - 1][y] = board[x][y] + 1;

visited[x - 1][y] = true;

bfsQueueX.push_back(x - 1); bfsQueueY.push_back(y);

}

if (y > 0 && board[x][y - 1] == 0 && !visited[x][y - 1]) {

board[x][y - 1] = board[x][y] + 1;

visited[x][y - 1] = true;

bfsQueueX.push_back(x); bfsQueueY.push_back(y - 1);

}

if (x < N - 1 && board[x + 1][y] == 0 && !visited[x + 1][y]) {

board[x + 1][y] = board[x][y] + 1;

visited[x + 1][y] = true;

bfsQueueX.push_back(x + 1); bfsQueueY.push_back(y);

}

if (y < M - 1 && board[x][y + 1] == 0 && !visited[x][y + 1]) {

board[x][y + 1] = board[x][y] + 1;

visited[x][y + 1] = true;

bfsQueueX.push_back(x); bfsQueueY.push_back(y + 1);

}

bfs();

}

int main()

{

int i, j;

cin >> M >> N;

for (i = 0; i < N; i++) {

for (j = 0; j < M; j++) {

cin >> board[i][j];

visited[i][j] = false;

}

}

for (i = 0; i < N; i++) {

for (j = 0; j < M; j++) {

if (board[i][j] == 1) {

bfsQueueX.push_back(i);

bfsQueueY.push_back(j);

visited[i][j] = true;

}

}

}

bfs();

for (i = 0; i < N; i++) {

for (j = 0; j < M; j++) {

if (board[i][j] == 0) {

days = 0; i = N;

break;

}

if (board[i][j] != -1 && board[i][j] > days) {

days = board[i][j];

}

}

}

cout << days - 1;

return 0;

}