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コード#コード#

from collections import deque
import sys
input = sys.stdin.readline

M, N = map(int, input().split())
tmt = [list(map(int, input().split())) for _ in range(N)]
dir = [(-1,0), (1,0), (0,-1), (0,1)]
q = deque()
for i in range(N):
    for j in range(M):
        if tmt[i][j] == 1:
            q.append((i, j))
while q:
    i, j = q.popleft()
    for k in range(4):
        ni, nj = i+dir[k][0], j+dir[k][1]
        if 0<=ni<N and 0<=nj<M and tmt[ni][nj] == 0:
            tmt[ni][nj] = tmt[i][j] + 1
            q.append((ni,nj))
is_ripe = True
mx = 0
for i in tmt:
    if 0 in i:
        is_ripe = False
        break
    if mx < max(i):
        mx = max(i)
if is_ripe:
    print(mx-1)
else:
    print(-1)

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