#include <bits/stdc++.h>
using namespace std;
const int inf = 1009;
int dx[4] = {-1, 1, 0, 0};
int dy[4] = {0, 0, -1, 1};
int b[inf][inf];
bool vis[inf][inf];
int main() {
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n, m; cin >> n >> m;
swap(n, m);
queue<pair<int, pair<int, int>>> q;
for (int i = 0; i < n; i++) for (int j = 0; j < m; j++) {
cin >> b[i][j];
if (b[i][j] == 1) {
q.push(make_pair(0, make_pair(i, j)));
vis[i][j] = 1;
}
}
int ans = 0;
while (!q.empty()) {
int dist = q.front().first;
ans = max(ans, dist);
pair<int, int> here = q.front().second;
int y = here.first;
int x = here.second;
q.pop();
for (int i = 0; i < 4; i++) {
int ny = y + dy[i];
int nx = x + dx[i];
if (ny >= n || ny < 0 || nx >= m || nx < 0 || vis[ny][nx] || b[ny][nx] == -1 || b[ny][nx] == 1) continue;
vis[ny][nx] = true;
q.push(make_pair(dist + 1, make_pair(ny, nx)));
}
}
int ok = true;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (vis[i][j] || b[i][j] == -1) continue;
ok = false;
}
}
if (!ok) ans = -1;
cout << ans << '\n';
}
