def main():
    import sys
    input = sys.stdin.read
    data = input().split()
    idx = 0
    H = int(data[idx])
    idx += 1
    W = int(data[idx])
    idx += 1
    S = []
    for _ in range(H):
        S.append(data[idx])
        idx += 1
    
    # Compute prefix sum of the grid
    sum_grid = [[0]*(W+2) for _ in range(H+2)]
    for i in range(1, H+1):
        for j in range(1, W+1):
            sum_grid[i][j] = sum_grid[i-1][j] + sum_grid[i][j-1] - sum_grid[i-1][j-1]
            if S[i-1][j-1] == '#':
                sum_grid[i][j] += 1
    
    def check(k):
        if k == 0 or k > H or k > W:
            return False
        
        # Compute valid grid
        valid = [[0]*(W+2) for _ in range(H+2)]
        for i in range(1, H+1):
            for j in range(1, W+1):
                if i + k - 1 > H or j + k - 1 > W:
                    continue
                a = i
                b = i + k - 1
                c = j
                d = j + k - 1
                total = sum_grid[b][d] - sum_grid[a-1][d] - sum_grid[b][c-1] + sum_grid[a-1][c-1]
                if total == k * k:
                    valid[i][j] = 1
        
        # Compute prefix sum of valid
        pre_valid = [[0]*(W+2) for _ in range(H+2)]
        for i in range(1, H+1):
            row_sum = 0
            for j in range(1, W+1):
                row_sum += valid[i][j]
                pre_valid[i][j] = pre_valid[i-1][j] + row_sum
        
        # Check each black cell
        for x in range(1, H+1):
            for y in range(1, W+1):
                if S[x-1][y-1] == '#':
                    a = max(1, x - k + 1)
                    b = x
                    c = max(1, y - k + 1)
                    d = y
                    if a > H or c > W or b < a or d < c:
                        return False
                    sum_rect = pre_valid[b][d] - pre_valid[a-1][d] - pre_valid[b][c-1] + pre_valid[a-1][c-1]
                    if sum_rect == 0:
                        return False
        return True
    
    low = 1
    high = min(H, W)
    ans = 0
    while low <= high:
        mid = (low + high) // 2
        if check(mid):
            ans = mid
            low = mid + 1
        else:
            high = mid - 1
    print(ans)

if __name__ == "__main__":
    main()