Problem:
Problem B: Cleaning Robot

Dr. Asimov, a robotics researcher, loves to research, but hates houseworks and his house were really dirty. So, he has developed a cleaning robot.

As shown in the following figure, his house has 9 rooms, where each room is identified by an alphabet:

The robot he developed operates as follows:

If the battery runs down, the robot stops.

If not so, the robot chooses a direction from four cardinal points with the equal probability, and moves to the room in that direction. Then, the robot clean the room and consumes 1 point of the battery.

However, if there is no room in that direction, the robot does not move and remains the same room. In addition, there is a junk room in the house where the robot can not enter, and the robot also remains when it tries to enter the junk room. The robot also consumes 1 point of the battery when it remains the same place.

A battery charger for the robot is in a room. It would be convenient for Dr. Asimov if the robot stops at the battery room when its battery run down.

Your task is to write a program which computes the probability of the robot stopping at the battery room.

Input

The input consists of several datasets. Each dataset consists of:

n
s t b

n is an integer that indicates the initial battery point. s, t, b are alphabets respectively represent the room where the robot is initially, the battery room, and the junk room.

The input ends with a dataset which consists of single 0 for n. Your program should not output for this dataset.

Output

For each dataset, print the probability as a floating point number in a line. The answer may not have an error greater than 0.000001.

Constraints

Judge data includes at most 100 data sets.

n ≤ 15

s, t, b are distinct.

Sample Input

1
E A C
1
E B C
2
E A B
0

Output for the Sample Input

0.00000000
0.25000000
0.06250000

Solution in JAVA (judge verdict: Wrong Answer)

import java.io.BufferedReader;
import java.io.InputStreamReader;

class Main{
	static double before[][];
	static double after[][];
	static boolean souko[][];
	public static void main(String args[]){
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		try{
			while(true){
				int n = Integer.parseInt(br.readLine());
				if(n == 0) return;

				String strs[] = br.readLine().split(" ");
				char s = strs[0].charAt(0);
				char t = strs[1].charAt(0);
				char b = strs[2].charAt(0);

				before = new double[3][3];
				souko = new boolean[3][3];

				int ts[] = charToInts(b);
				souko[ts[0]][ts[1]] = true;
				//print(souko);

				int ss[] = charToInts(s);
				before[ss[0]][ss[1]] = 1.0;
				//print(before);

				for(int i = 0; i < n; i++){
					after = new double[3][3];
					calc();
					before = after;
					//print(before);
				}

				int tts[] = charToInts(t);
				System.out.println(before[ts[0]][ts[1]]);
			}
		}catch(Exception e){
		}
	}

	static void calc(){
		for(int i = 0; i < 3; i++){
			for(int j = 0; j < 3; j++){
				double p = before[i][j] * 0.25;
				// ←
				if(j == 0 || souko[i][j-1]){
					after[i][j] += p;
				}else{
					after[i][j-1] += p;
				}

				// ↑
				if(i == 0 || souko[i-1][j]){
					after[i][j] += p;
				}else{
					after[i-1][j] += p;
				}

				// →
				if(j == 2 || souko[i][j+1]){
					after[i][j] += p;
				}else{
					after[i][j+1] += p;
				}

				// ↓
				if(i == 2 || souko[i+1][j]){
					after[i][j] += p;
				}else{
					after[i+1][j] += p;
				}
			}
		}
	}

	static int[] charToInts(char c){
		int m = c - 'A';
		System.out.println(m);
		int result[] = {m / 3, m % 3};
		System.out.println("(" + result[0] + ", " + result[1] + ")");
		return result;
	}

	static void print(double m[][]){
		for(int i = 0; i < m.length; i++){
			for(int j = 0; j < m[i].length; j++){
				System.out.print(m[i][j] + "\t");
			}
			System.out.println();
		}
	}

	static void print(boolean m[][]){
		for(int i = 0; i < m.length; i++){
			for(int j = 0; j < m[i].length; j++){
				System.out.print(m[i][j] + "\t");
			}
			System.out.println();
		}
	}
}