### maxDistance 
from collections import deque
from typing import *
def f_gold(grid: List[List[int]]) -> int:
    n = len(grid)
    q = deque([(i, j) for i in range(n) for j in range(n) if grid[i][j] == 1])
    ans = -1
    valid = False
    while q:
        ans += 1
        for _ in range(len(q)):
            i, j = q.popleft()
            for a, b in [[0, 1], [0, -1], [1, 0], [-1, 0]]:
                x, y = i + a, b + j
                if 0 <= x < n and 0 <= y < n and grid[x][y] == 0:
                    valid = True
                    grid[x][y] = 1
                    q.append((x, y))
    return ans if valid else -1