s3 (CCO Canadian_Computing_Competition_Senior)
Time limit: 1 seconds | Memory limit: 512 MB | Task type: batch
Algorithms: case analysis, constructive

# Problem S3: Palindromic Poster

### Problem Description

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-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.

### Input Specification

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 | $2 \le N \le 2000$ | $2 \le M \le 2000$ | $R = 1$         | $C = 1$         |
| 2 marks | $N = 2$            | $M = 2$            | $0 \le R \le N$ | $0 \le C \le M$ |
| 4 marks | $N = 2$            | $2 \le M \le 2000$ | $0 \le R \le N$ | $0 \le C \le M$ |
| 7 marks | $2 \le N \le 2000$ | $2 \le M \le 2000$ | $0 \le R \le N$ | $0 \le C \le M$ |

### Output Specification

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 of them.

### Sample Input 1

```
4 5 1 2
```

### Output for Sample Input 1

```
union
radar
badge
anime
```

### 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.

### Sample Input 2

```
2 2 2 1
```

### Output for Sample Input 2

```
IMPOSSIBLE
```

### Explanation of Output for Sample Input 2

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

Starter code:

Grader code:

Setup script:

Evaluation script: