import sys


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    w, h = map(int, input().split())

    def encode(i, j):
        return i * w + j

    c = [input() for _ in range(h)]
    d = [[float('inf')] * (h * w) for _ in range(h * w)]
    for i in range(h * w):
        d[i][i] = 0
    for i in range(h):
        for j in range(w):
            if j != 0:
                d[encode(i, j)][encode(i, j - 1)] = d[encode(i, j - 1)][encode(i, j)] = \
                    1 if c[i][j] == '#' or c[i][j - 1] == '#' else 0
            if i != 0:
                d[encode(i, j)][encode(i - 1, j)] = d[encode(i - 1, j)][encode(i, j)] = \
                    1 if c[i][j] == '#' or c[i - 1][j] == '#' else 0
    for k in range(h * w):
        for i in range(h * w):
            for j in range(h * w):
                d[i][j] = min(d[i][j], d[i][k] + d[k][j])
    ans = 0
    for i in range(h):
        for j in range(w):
            for k in range(h):
                for s in range(w):
                    if c[i][j] == '.' and c[k][s] == '.':
                        ans = max(ans, d[encode(i, j)][encode(k, s)])
    return ans - 1


if __name__ == '__main__':
    ret = main()
    if ret is not None:
        print(ret)