#include <cstdio>
#include <cstring>const int maxn = 1005, maxe = 1005;int n, e, ini_neko, ini_mouse, t1, t2;
int head[maxn], to[maxe << 1], next[maxe << 1], lb, out[maxn];
int que[maxn], head_, tail, h, x[maxn][maxn], d[maxn][maxn];
double f[maxn][maxn];inline void ist(int aa, int ss) {to[lb] = ss;next[lb] = head[aa];head[aa] = lb;++out[aa];++lb;
}
double dp(int i, int j) {if (f[i][j] != -1.0) {return f[i][j];}if (i == j) {return f[i][j] = 0.0;}if (x[i][j] == j) {return f[i][j] = 1.0;}if (x[x[i][j]][j] == j) {return f[i][j] = 1.0;}f[i][j] = 0.0;for (int k = head[j]; k != -1; k = next[k]) {f[i][j] += dp(x[x[i][j]][j], to[k]);}f[i][j] = (f[i][j] + dp(x[x[i][j]][j], j)) / (double)(out[j] + 1) + 1;return f[i][j];
}int main(void) {//freopen("in.txt", "r", stdin);memset(head, -1, sizeof head);memset(next, -1, sizeof next);for (int i = 0; i < maxn; ++i) {for (int j = 0; j < maxn; ++j) {f[i][j] = -1.0;}}scanf("%d%d", &n, &e);scanf("%d%d", &ini_neko, &ini_mouse);while (e--) {scanf("%d%d", &t1, &t2);ist(t1, t2);ist(t2, t1);}memset(d, -1, sizeof d);for (int i = 1; i <= n; ++i) {memset(que, 0, sizeof que);head_ = tail = 0;que[tail++] = i;d[i][i] = 0;while (head_ != tail) {h = que[head_++];for (int j = head[h]; j != -1; j = next[j]) {if (d[i][to[j]] == -1) {d[i][to[j]] = d[i][h] + 1;que[tail++] = to[j];}}}}memset(x, 0x3c, sizeof x);for (int i = 1; i <= n; ++i) {for (int k = head[i]; k != -1; k = next[k]) {for (int j = 1; j <= n; ++j) {if (d[i][j] == d[to[k]][j] + 1 && x[i][j] > to[k]) {x[i][j] = to[k];}}}}printf("%.3f\n", dp(ini_neko, ini_mouse));return 0;
}