#include <iostream>
#include <queue>
#include <utility>
using namespace std;
using pi = pair<int,int>;
#define F first
#define S second
#define isInBoard(r,c) (r >= 0 && r < n && c >= 0 && c < m)
const int maxn = 1000, maxm = 1000;
int n, m, ans, tm[maxn][maxm], days[maxn][maxm];
bool visited[maxn][maxm], isThereNotRiped;
queue<pi> q;
void bfs(pi s, pi e) {
if (isInBoard(e.F, e.S) && !visited[e.F][e.S] && !tm[e.F][e.S]) {
visited[e.F][e.S] = true;
days[e.F][e.S] = days[s.F][s.S]+1;
q.push({e.F, e.S});
ans = max(ans, days[e.F][e.S]);
}
return;
}
int main() {
ios::sync_with_stdio(0); cin.tie(0);
cin >> m >> n;
for (int r = 0; r < n; ++r) {
for (int c = 0; c < m; ++c) {
cin >> tm[r][c];
if (!tm[r][c]) isThereNotRiped = true;
else if (tm[r][c] == 1) {
q.push({r, c});
days[r][c] = 0; visited[r][c] = true;
}
}
}
if (!isThereNotRiped) {
cout << 0;
return 0;
}
while (!q.empty()) {
pi s = q.front(); q.pop();
tm[s.F][s.S] = 1;
bfs(s, {s.F, s.S+1});
bfs(s, {s.F+1, s.S});
bfs(s, {s.F, s.S-1});
bfs(s, {s.F-1, s.S});
}
for (int r = 0; r < n; ++r) for (int c = 0; c < m; ++c) {
if (!tm[r][c]) {
cout << -1;
return 0;
}
}
cout << ans;
return 0;
}
