1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<algorithm>
 5 #include<vector>
 6 using namespace std;
 7 const int maxn=985,maxm=985;
 8 vector<int>G[maxn];
 9 int du[maxn];
10 double f[maxn][maxn];
11 int n,e,c,m;
12 #include<queue>
13 int near[maxn][maxn];
14 void Spfa(int x)
15 {
16     bool vis[maxn]={0};
17     queue<int>q;
18     near[x][x]=x;
19     q.push(x);vis[x]=1;
20     while(!q.empty())
21     {
22         int t=q.front();q.pop();
23         for(int i=0;i<G[t].size();i++)
24         {
25             int v=G[t][i];
26             if(vis[v])continue;
27             if(t==x)near[x][v]=v;
28             else near[x][v]=near[x][t];
29             vis[v]=1;
30             q.push(v);
31         }
32     }
33 }
34 const double eps=1e-6;
35 double dp(int from,int to)
36 {
37     if(f[from][to]>eps)return f[from][to];
38     if(from==to)return 0;
39     int go=near[near[from][to]][to];
40     if(go==to)return f[from][to]=1;
41     double ans=dp(go,to);
42     for(int i=0;i<G[to].size();i++)
43     {
44         int v=G[to][i];
45         ans+=dp(go,v);
46     }
47     ans/=(double)(du[to]+1);
48     ans++;
49     return f[from][to]=ans;
50 }
51 int haha()
52 {
53     //freopen("cchkk.in","r",stdin);
54     //freopen("cchkk.out","w",stdout);
55     scanf("%d%d",&n,&e);scanf("%d%d",&c,&m);
56     for(int i=1;i<=e;i++)
57     {
58         int x,y;scanf("%d%d",&x,&y);
59         G[x].push_back(y);G[y].push_back(x);
60         du[x]++;du[y]++;
61     }
62     for(int i=1;i<=n;i++)sort(G[i].begin(),G[i].end());
63     for(int i=1;i<=n;i++)Spfa(i);
64     printf("%0.3lf",dp(c,m));
65 }
66 int sb=haha();
67 int main(){;}