CF510B.Fox And Two Dots

Fox Ciel is playing a mobile puzzle game called "Two Dots". The basic levels are played on a board of size n × m cells, like this:

Each cell contains a dot that has some color. We will use different uppercase Latin characters to express different colors.

The key of this game is to find a cycle that contain dots of same color. Consider 4 blue dots on the picture forming a circle as an example. Formally, we call a sequence of dots _d_1, _d_2, ..., d__k a cycle if and only if it meets the following condition:

1. These k dots are different: if i ≠ j then d__i is different from d__j.
2. k is at least 4.
3. All dots belong to the same color.
4. For all 1 ≤ i ≤ k - 1: d__i and d__i + 1 are adjacent. Also, d__k and _d_1 should also be adjacent. Cells x and y are called adjacent if they share an edge.

Determine if there exists a cycle on the field.

狐狸雪儿正在玩一款名为“两点”的手机益智游戏。基础关卡在一个 $n \times m$ 的方格棋盘上进行，如下图所示：

每个格子中包含一个带有某种颜色的点。我们使用不同的大写拉丁字母来表示不同的颜色。

该游戏的关键在于找出一个由同色点构成的环。例如，图中四个蓝色点构成一个环。形式化地，我们称点序列 $d_1, d_2, \dots, d_k$ 为一个环，当且仅当它满足以下条件：

1. 这 $k$ 个点互不相同：若 $i \ne j$，则 $d_i \ne d_j$；
2. $k \ge 4$；
3. 所有点颜色相同；
4. 对所有 $1 \le i \le k-1$，$d_i$ 与 $d_{i+1}$ 相邻；且 $d_k$ 与 $d_1$ 也相邻。若两个格子 $x$ 与 $y$ 共享一条边，则称它们相邻。

请判断棋盘上是否存在这样的环。

The first line contains two integers n and m (2 ≤ n, m ≤ 50): the number of rows and columns of the board.

Then n lines follow, each line contains a string consisting of m characters, expressing colors of dots in each line. Each character is an uppercase Latin letter.

第一行包含两个整数 $n$ 和 $m$（$2 \leq n, m \leq 50$）：棋盘的行数和列数。

接下来有 $n$ 行，每行包含一个长度为 $m$ 的字符串，表示该行中各点的颜色。每个字符均为大写拉丁字母。

Output "Yes" if there exists a cycle, and "No" otherwise.

如果存在环，则输出“Yes”，否则输出“No”。

In first sample test all 'A' form a cycle.

In second sample there is no such cycle.

The third sample is displayed on the picture above ('Y' = Yellow, 'B' = Blue, 'R' = Red).

在第一个样例测试中，所有 'A' 构成一个环。

在第二个样例中，不存在这样的环。

第三个样例显示在上方图片中（'Y' = 黄色，'B' = 蓝色，'R' = 红色）。