#include<cstdio>
#include<queue>
using namespace std;

int encode(int n, int r, int c)
{
	return r * n + c;
}

void decode(int n, int id, int& r, int& c)
{
	r = id / n;
	c = id % n;
	return;
}

int solve(int n, int **maze)
{
	int ret = 0;
	queue<int> q;
	int *dis = new int [n * n];
	const int dr[] = {-1, 1, 0, 0}, dc[] = {0, 0, -1, 1};
	
	for (int i = 0; i < n; ++i)
	{
		for (int j = 0; j < n; ++j)
		{
			if (maze[i][j] == 1)
			{
				q.push(encode(n, i, j));
				dis[encode(n, i, j)] = 0;
			}
			else
			{
				dis[encode(n, i, j)] = -1;
			}
		}
	}
	
	while (q.size())
	{
		int u = q.front(); q.pop();
		int r, c;
		
		decode(n, u, r, c);
		
		ret = max(ret, dis[u]);
		
		for (int k = 0; k < 4; ++k)
		{
			int nr = r + dr[k], nc = c + dc[k];
			if (0 <= nr && nr < n && 0 <= nc && nc < n && maze[nr][nc] != -1 && dis[encode(n, nr, nc)] == -1)
			{
				q.push(encode(n, nr, nc));
				dis[encode(n, nr, nc)] = dis[encode(n, r, c)] + 1;
			}
		}
	}
	
	delete [] dis;
	
	return ret;
}

int main()
{
	int n;
	
	scanf("%d", &n);
	
	int **maze = new int* [n];
	
	for (int i = 0; i < n; ++i)
	{
		maze[i] = new int[n];
		for (int j = 0; j < n; ++j)
		{
			scanf("%d", &maze[i][j]);
		}
	}
		
	int ans = solve(n, maze);
	
	printf("%d\n", ans);
		
	for (int i = 0; i < n; ++i)
	{
		delete [] maze[i];
	}
	delete [] maze;
	
	return 0;
}