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

#define MAX 100

int N, V, T1, T2;
int adj[MAX][MAX], nadj[MAX];
int cur;
double mat[2][MAX][MAX];

void init() {
	memset(nadj, 0, sizeof(nadj));
	cur = 0;
	memset(mat[cur], 0, sizeof(mat[cur]));
}

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[cur][i][i] = 1.0;
		}
		for (p = 0; p < nadj[i]; p++) {
			j = adj[i][p];
			if (j == T1 || j == T2) {
				continue;
			}
			mat[cur][i][j] = 1.0 / nadj[j];
		}
	}
}

void sqr_matrix() {
	int i, j, k;
	memset(mat[!cur], 0, sizeof(mat[!cur]));	
	for (i = 0; i < N; i++) {
		for (j = 0; j < N; j++) {
			for (k = 0; k < N; k++) {
				mat[!cur][i][j] += mat[cur][i][k] * mat[cur][k][j];
			}
		}
	}
	cur = !cur;
}

int main() {
	int s;
	while (scanf(" %d %d %d %d", &N, &V, &T1, &T2) != EOF) {
		V--; T1--; T2--;
		init();
		read_graph();
		gen_matrix();
		for (s = 0; s < 100; s++) {
			sqr_matrix();
		}
		printf("%.6f\n", fabs(mat[cur][T1][V]));
	}
	return 0;
}