// HoaiNam
#include <bits/stdc++.h>
#define endl "\n"
#define int long long
#define all(x) x.begin(), x.end()
#define NAME ""
using namespace std;

const int N = 1e3 + 1;
const int INF = 1e9;
const int mod = 1e9 + 7;

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

int n, m, dist[N][N];
char A[N][N];

void bfs() {
    queue<pair<int, int>> q;
    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= m; ++j) {
            dist[i][j] = INF;
        }
    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= m; ++j) {
            if (A[i][j] == '1') {
                q.push({i, j});
                dist[i][j] = 0;
            }
        }

    while (!q.empty()) {
        auto [u, v] = q.front();
        q.pop();
        for (int i = 0; i < 4; ++i) {
            int x = u + dx[i];
            int y = v + dy[i];
            if (x >= 1 && x <= n && y >= 1 && y <= m && dist[x][y] > dist[u][v] + 1) {
                dist[x][y] = dist[u][v] + 1;
                q.push({x, y});
            }
        }
    }
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    if (fopen(NAME".INP", "r")) {
        freopen(NAME".INP", "r", stdin);
        freopen(NAME".OUT", "w", stdout);
    }

    cin >> n >> m;

    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            cin >> A[i][j];
        }
    }

    bfs();

    int ans = 0;
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            if (dist[i][j] != INF) ans = max(ans, dist[i][j]);
        }
    }
    cout << ans;

    // HoaiNam
}