#include <stdio.h>
#include <string.h>
#include <math.h>

#define MAX 100

int N, V, T1, T2;
int adj[MAX][MAX], nadj[MAX];
double mat[MAX][MAX];
double vec[MAX];

void init() {
	memset(nadj, 0, sizeof(nadj));
	memset(mat, 0, sizeof(mat));
	memset(vec, 0, sizeof(vec));
}

void read_graph() {
	int i;
	int X, Y;
	for (i = 0; i < N-1; i++) {
		scanf(" %d %d", &X, &Y);
		X--; Y--;
		adj[X][nadj[X]++] = Y;
		adj[Y][nadj[Y]++] = X;
	}
}

void gen_matrix() {
	int i, p, j;
	for (i = 0; i < N; i++) {
		if (i == T1 || i == T2) {
			mat[i][i] = 1.0;
			continue;
		}
		mat[i][i] = -1.0;
		for (p = 0; p < nadj[i]; p++) {
			j = adj[i][p];
			mat[i][j] = 1.0 / nadj[i];
		}
	}
	vec[T1] = 1.0;
}

void find_eigenvector() {
	int i, j;
	int r, p;
	for (r = 0; r < N; r++) {
		for (p = r; p < N; p++) {
			if (fabs(mat[p][r]) > 1e-10) {
				break;
			}
		}
		if (p == N) {
			continue;
		}
		for (j = 0; j < N; j++) {
			double t = mat[r][j];
			mat[r][j] = mat[p][j];
			mat[p][j] = t;
		}
		{
			double t = vec[r];
			vec[r] = vec[p];
			vec[p] = t;
		}
		for (i = r+1; i < N; i++) {
			double t = mat[i][r] / mat[r][r];
			for (j = r; j < N; j++) {
				mat[i][j] -= t * mat[r][j];
			}
			vec[i] -= t * vec[r];
		}
	}
	for (r = N-1; r >= 0; r--) {
		double t = mat[r][r];
		for (j = 0; j < N; j++) {
			mat[r][j] /= t;
		}
		vec[r] /= t;
		for (i = 0; i < r; i++) {
			double t = mat[i][r];
			for (j = 0; j < N; j++) {
				mat[i][j] -= t * mat[r][j];
			}
			vec[i] -= t * vec[r];
		}
	}
}

int main() {
	while (scanf(" %d %d %d %d", &N, &V, &T1, &T2) != EOF) {
		V--; T1--; T2--;
		init();
		read_graph();
		gen_matrix();
		find_eigenvector();
		printf("%.6f\n", fabs(vec[V]));
	}
	return 0;
}