h,w = map(int,input().split())
s = [[int(v=="#") for v in input()]+[0] for _ in range(h)]+[[0]*(w+1)]
t = [si[:] for si in s]
for i in range(h):
    for j in range(w-1): t[i][j+1] += t[i][j]
for j in range(w):
    for i in range(h-1): t[i+1][j] += t[i][j]
ok = 1; ng = min(h,w)+1
while ng-ok>1:
    mid = (ok+ng)//2
    f = [[0]*(w+1) for _ in range(h+1)]
    for i in range(h-mid+1):
        for j in range(w-mid+1):
            if t[i+mid-1][j+mid-1]-t[i+mid-1][j-1]-t[i-1][j+mid-1]+t[i-1][j-1]==mid**2:
                f[i][j] += 1; f[i][j+mid] -= 1; f[i+mid][j] -= 1; f[i+mid][j+mid] += 1
    for i in range(h+1):
        for j in range(w): f[i][j+1] += f[i][j]
    for j in range(w+1):
        for i in range(h): f[i+1][j] += f[i][j]
    for i in range(h):
        for j in range(w):
            if f[i][j]: f[i][j] = 1
    if f==s: ok = mid
    else: ng = mid
print(ok)