from collections import deque
import sys

input = sys.stdin.readline

M, N = map(int, input().split())
emptyCount = 0
tomatoCount = 0
visited = [[-1]*M for _ in range(N)]
q = deque()

for n in range(N) :
    row = list(map(int, input().split()))
    for m in range(M) :
        tomato = row[m]
        if tomato == 1 :
            q.append((m,n))
            visited[n][m] = 0
            tomatoCount += 1
        elif tomato == -1 :
            visited[n][m] = -2
            emptyCount += 1

dm = [1,-1,0,0]
dn = [0,0,1,-1]
dayMax = 0

while len(q):
    cm, cn = q.popleft()
    
    for i in range(4) :
        nm, nn = cm + dm[i], cn + dn[i]
        if 0 <= nm < M and 0 <= nn < N and visited[nn][nm] == -1 :
            q.append((nm,nn))
            tomatoCount += 1
            visited[nn][nm] = visited[cn][cm] + 1
            dayMax = max(dayMax, visited[nn][nm])

print(dayMax if tomatoCount + emptyCount == N*M else -1)