Problem:
Score: 400 points

Problem Statement

Amidakuji is a traditional method of lottery in Japan.

To make an amidakuji, we first draw W parallel vertical lines, and then draw horizontal lines that connect them. The length of each vertical line is H+1 [cm], and the endpoints of the horizontal lines must be at 1, 2, 3, ..., or H [cm] from the top of a vertical line.

A valid amidakuji is an amidakuji that satisfies the following conditions:

No two horizontal lines share an endpoint.

The two endpoints of each horizontal lines must be at the same height.

A horizontal line must connect adjacent vertical lines.

Find the number of the valid amidakuji that satisfy the following condition, modulo 1\ 000\ 000\ 007: if we trace the path from the top of the leftmost vertical line to the bottom, always following horizontal lines when we encounter them, we reach the bottom of the K-th vertical line from the left.

For example, in the following amidakuji, we will reach the bottom of the fourth vertical line from the left.

Constraints

H is an integer between 1 and 100 (inclusive).

W is an integer between 1 and 8 (inclusive).

K is an integer between 1 and W (inclusive).

Input

Input is given from Standard Input in the following format:

H W K

Output

Print the number of the amidakuji that satisfy the condition, modulo 1\ 000\ 000\ 007.

Sample Input 1

1 3 2

Sample Output 1

1

Only the following one amidakuji satisfies the condition:

Sample Input 2

1 3 1

Sample Output 2

2

Only the following two amidakuji satisfy the condition:

Sample Input 3

2 3 3

Sample Output 3

1

Only the following one amidakuji satisfies the condition:

Sample Input 4

2 3 1

Sample Output 4

5

Only the following five amidakuji satisfy the condition:

Sample Input 5

7 1 1

Sample Output 5

1

As there is only one vertical line, we cannot draw any horizontal lines. Thus, there is only one amidakuji that satisfies the condition: the amidakuji with no horizontal lines.

Sample Input 6

15 8 5

Sample Output 6

437760187

Be sure to print the answer modulo 1\ 000\ 000\ 007.

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
import java.util.stream.*;

public class Main {
  public static int MOD = 1_000_000_007;

  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    int H = in.nextInt(), W = in.nextInt(), K = in.nextInt();
    int[][] dp = new int[H + 1][W];
    dp[0][0] = 1;
    for (int i = 0; i < H; i++) {
      for (int k = 0; k < Math.pow(2, W - 1); k++) {
        int val = k;
        int[] arr = new int[W - 1];
        for (int j = 0; j < W - 1; j++) {
          arr[j] = val % 2;
          val /= 2;
        }
        boolean valid = true;
        for (int j = 1; j < W - 1; j++) {
          if (arr[j - 1] == 1 && arr[j] == 1) {
            valid = false;
          }
        }
        if (!valid) {
          continue;
        }
        for (int j = 0; j < W; j++) {
          if (j - 1 >= 0 && arr[j - 1] == 1) {
            dp[i + 1][j - 1] += dp[i][j];
          }
          if (j + 1 < W && arr[j] == 1) {
            dp[i + 1][j + 1] += dp[i][j];
          }
          boolean left = j == 0 ? true : arr[j - 1] == 0;
          boolean right = j == W - 1 ? true : arr[j] == 0;
          if (left && right) {
            dp[i + 1][j] += dp[i][j];
          }
        }
        for (int j = 0; j < W; j++) {
          dp[i + 1][j] %= MOD;
        }
      }
    }
    System.out.println(dp[H][K - 1] % MOD);
  }
}