211009

tomatobox = []
findlist = []
countday = 0
f = 0
r = 0

xnum, ynum = map(int, input().split())
x = [-1]*(xnum+2)
tomatobox.append(x)
for i in range(ynum): # 토마토 박스 만들귀
    a = []
    a.append(-1)
    data = list(map(int, input().split()))
    for x in range(len(data)):
        a.append(data[x])

    a.append(-1)
    tomatobox.append(a)
x = [-1]*(xnum+2)
tomatobox.append(x)

def findone(): # 숫자 1의 좌표값 찾기
    global findlist
    global f
    findlist = []
    for y in range(1, ynum+2):
        for x in range(1, xnum+2):
            if tomatobox[y][x] == 1:
                onenum = [y, x]
                findlist.append(onenum)

def findunripe(num):
    global f
    x = num[0]
    y = num[1]

    # print(tomatobox[x+1][y])
    # print(tomatobox[x-1][y])
    # print(tomatobox[x][y+1])
    # print(tomatobox[x][y-1])
    if (tomatobox[x+1][y] == 0):
        tomatobox[x+1][y] = 1
        findlist.append([x+1, y])
        f += 1

    if (tomatobox[x-1][y] == 0):
        tomatobox[x-1][y] = 1
        findlist.append([x-1, y])
        f += 1

    if (tomatobox[x][y+1] == 0):
        tomatobox[x][y+1] = 1
        findlist.append([x, y+1])
        f += 1

    if (tomatobox[x][y-1] == 0):
        tomatobox[x][y-1] = 1
        findlist.append([x, y-1])
        f += 1

def ripe():
    global countday
    global r
    forcount = 0
    while True:
        if findlist.count(0) == len(findlist):
            break
        else:
            for i in findlist:
                if forcount == 0:
                    forcount = len(findlist) - findlist.count(0)
                    countday += 1
                forcount -= 1
                findunripe(i)
                findlist[r] = 0
                r += 1
#    print(countday-1)

def cantripe():
    for y in range(1, ynum + 2):
        for x in range(1, xnum + 2):
            if tomatobox[y][x] == 0:
                return True
isallripe = False
def allripe():
    global isallripe
    for y in range(1, ynum + 2):
        for x in range(1, xnum + 2):
            if tomatobox[y][x] == 1 or tomatobox[y][x] == -1:
                isallripe = True
            else:
                return False
    return isallripe

findone()

if allripe():
    print(0)
else:
    ripe()
    if cantripe():
        print(-1)
    else:
        print(countday-1)