题目描述

Ryo and Kita are designing a new poster for Kessoku Band. After some furious brainstorming, they came to the conclusion that the poster should come in the form of a $2\text{-D}$ grid of lowercase English letters (i.e. a to z), with $N$ rows and $M$ columns.

Furthermore, it is known that Ryo and Kita both have peculiar tastes in palindromes. Ryo will only be satisfied with the poster if exactly $R$ of its rows are palindromes, and Kita will only be satisfied with the poster if exactly $C$ of its columns are palindromes. Can you design a poster that will satisfy both Ryo and Kita, or determine that it is impossible to do so?

Note: A string is considered a palindrome if it is the same when read forwards and backwards. For example, kayak and bb are palindromes, whereas guitar and live are not.

输入格式

The first and only line of input consists of $4$ space-separated integers $N, M, R$, and $C$.

The following table shows how the available $15$ marks are distributed.

Marks	Bounds on $N$	Bounds on $M$	Bounds on $R$	Bounds on $C$
$2$ marks	$2 \leq N \leq 2000$	$2 \leq M \leq 2000$	$R = 1$	$C = 1$
	$N = 2$	$M = 2$	$0 \leq R \leq N$	$0 \leq C \leq M$
$4$ marks		$2 \leq M \leq 2000$
$7$ marks	$2 \leq N \leq 2000$

输出格式

If it is impossible to design a poster that will satisfy both Ryo and Kita, output IMPOSSIBLE on a single line.

Otherwise, your output should contain $N$ lines, each consisting of $M$ lowercase English letters, representing your poster design. If there are multiple possible designs, output any ofthem.

4 5 1 2

union
radar
badge
anime

2 2 2 1

IMPOSSIBLE

提示

Explanation of Output for Sample Input $1$：

In the given design, only the second row (namely radar) and the second and third columns (namely naan and iddi) are palindromes. Since exactly $R = 1$ of the rows and $C = 2$ of the columns are palindromes, this is an acceptable design.

Explanation of Output for Sample Input $2$：

In this case, it can be proven that it is impossible to satisfy both Ryo and Kita.

本题采用捆绑测试：

• Subtask 1（2 points）：数据保证 $2 \leq N \leq 2000$，$2\leq M\leq 2000$，$R = 1$，$C = 1$。

• Subtask 2（2 points）：数据保证 $N = 2$，$M = 2$，$0\leq R\leq N$，$0\leq C\leq M$。

• Subtask 3（4 points）：数据保证 $N = 2$，$2\leq M \leq2000$，$0\leq R\leq N$，$0\leq C\leq M$。

• Subtask 4（7 points）：数据保证 $2\leq N\leq 2000$，$2\leq M \leq2000$，$0\leq R\leq N$，$0\leq C\leq M$。