h,w = map(int,input().split())
s = [input() for _ in range(h)]

ans = 0

for i in range(h):
    for j in range(w):
        if s[i][j] == '.':
            dis = [[-1] * w for _ in range(h)]
            dis[i][j] = 0
            q = [(i,j)]
            while q:
                l0,l1 = q.pop(0)
                if l0 > 0:
                    if s[l0-1][l1] == '.':
                        if dis[l0-1][l1] > dis[l0][l1] + 1 or dis[l0-1][l1] == -1:
                            dis[l0-1][l1] = dis[l0][l1] + 1
                            q.append((l0-1,l1))
                if l0 < h - 1:
                    if s[l0+1][l1] == '.':
                        if dis[l0+1][l1] > dis[l0][l1] + 1 or dis[l0+1][l1] == -1:
                            dis[l0+1][l1] = dis[l0][l1] + 1
                            q.append((l0+1,l1))
                if l1 > 0:
                    if s[l0][l1-1] == '.':
                        if dis[l0][l1-1] > dis[l0][l1] + 1 or dis[l0][l1-1] == -1:
                            dis[l0][l1-1] = dis[l0][l1] + 1
                            q.append((l0,l1-1))
                if l1 < w - 1:
                    if s[l0][l1+1] == '.':
                        if dis[l0][l1+1] > dis[l0][l1] + 1 or dis[l0][l1+1] == -1:
                            dis[l0][l1+1] = dis[l0][l1] + 1
                            q.append((l0,l1+1))
        dis_max = 0
        for k in dis:
            dis_max = max(max(k), dis_max)
        ans = max(dis_max,ans)

print(ans)
