#include <iostream>
#include <string>
#include <queue>
#include <algorithm>

using namespace std;
#define X first
#define Y second

int board[1002][1002];
//bool vis[102][102];
int day[1002][1002]={0};

int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    // 열(세로),행(가로)
    int n,m;
    cin >> m >> n;
    queue<pair<int,int>> q;
    for(int i=0; i<n; i++) {
        for(int j=0; j<m; j++) {
            cin >> board[i][j];
            if(board[i][j]==1) {
                q.push({i,j});   
            }
            //익지 않음
            else if(board[i][j]==0) {
                day[i][j]=-1;
            }
        }
    }


    while(!q.empty()) {
        pair<int,int> cur = q.front();
        q.pop();
        for(int i=0; i<4; i++) {
            int nx=cur.X+dx[i];
            int ny=cur.Y+dy[i];

            if(nx<0 || nx>=n || ny<0 || ny >= m) continue;
            //day가 -1인곳 탐색(방문하지 않음)
            //|| board[nx][ny]!=0
            if(day[nx][ny]>=0 ) continue;
            board[nx][ny]=1;
            day[nx][ny]=day[cur.X][cur.Y]+1;
            q.push({nx,ny});
        }
    }
    int res=0;
    for(int i=0; i<n; i++) {
        for(int j=0; j<m; j++) {
            if(board[i][j]==0) {
                cout << -1;
                return 0;
            }
            res=max(res,day[i][j]);
        }
    }
    cout << res;
}