import copy
from collections import deque


def bfs(que, field):
    field[que[0]][que[1]] = 0
    moves = [[1, 0], [-1, 0], [0, 1], [0, -1]]

    while que:
        c = que.popleft()
        for m in moves:
            my, mx = c[0] + m[0], c[1] + m[1]

            if field[my][mx] == ".":
                field[my][mx] = field[c[0]][c[1]] + 1
                que.append([my, mx])
            else:
                continue

    field = sum(field, [])
    return max([x for x in field if type(x) == int])


def main():
    H, W = map(int, input().split())
    field_s = []

    for h in range(H + 2):
        field_s.append(["#"] * (W + 2))
        if 1 <= h <= (H):
            field_s[h][1:-1] = list(map(str, input().rstrip()))

    start = []
    for y in range(1, H + 1):
        for x in range(1, W + 1):
            if field_s[y][x] == ".":
                start.append([y, x])

    ans = 0
    for s in start:
        que = deque([s])
        field = copy.deepcopy(field_s)
        ans = max(ans, bfs(que, field))

    print(ans)


if __name__ == '__main__':
    main()