bzoj1415

比较简单的数学期望，先预处理出当聪聪在i，可可在j时聪聪往哪个点走
然后做dp即可，我用了记忆化搜索实现

 type node=record
        po,next:longint;
      end;
 
 var d,pos:array[..,..] of longint;
     f:array[..,..] of double;
     v:array[..] of boolean;
     e:array[..] of node;
     c,p,q:array[..] of longint;
     len,s,t,i,j,n,m,x,y:longint;
 
 procedure add(x,y:longint);
   begin
     inc(len);
     e[len].po:=y;
     e[len].next:=p[x];
     p[x]:=len;
   end;
 
 procedure bfs(s:longint);
   var f,r,i,x,y:longint;
   begin
     d[s,s]:=;
     fillchar(v,sizeof(v),false);
     v[s]:=true;
     f:=;
     r:=;
     i:=p[s];
     while i<> do
     begin
       y:=e[i].po;
       v[y]:=true;
       inc(r);
       q[r]:=y;
       d[s,y]:=;
       pos[s,y]:=y;
       i:=e[i].next;
     end;
     while f<=r do
     begin
       x:=q[f];
       i:=p[x];
       while i<> do
       begin
         y:=e[i].po;
         if not v[y] then
         begin
           d[s,y]:=d[s,x]+;
           pos[s,y]:=pos[s,x];
           v[y]:=true;
           inc(r);
           q[r]:=y;
         end
         else if (d[s,y]=d[s,x]+) and (pos[s,x]<pos[s,y]) then
           pos[s,y]:=pos[s,x];
 
         i:=e[i].next;
       end;
       inc(f);
     end;
   end;
 
 function dfs(x,y:longint):double;
   var i,w:longint;
   begin
     if x=y then exit();
     if (pos[x,y]=y) or (pos[pos[x,y],y]=y) then exit();
     if f[x,y]<>- then exit(f[x,y]);
     i:=p[y];
     f[x,y]:=dfs(pos[pos[x,y],y],y);
     while i<> do
     begin
       w:=e[i].po;
       f[x,y]:=f[x,y]+dfs(pos[pos[x,y],y],w);
       i:=e[i].next;
     end;
     f[x,y]:=f[x,y]/(c[y]+)+;
     exit(f[x,y]);
   end;
 
 begin
   readln(n,m);
   readln(s,t);
   for i:= to m do
   begin
     readln(x,y);
     add(x,y);
     add(y,x);
     inc(c[x]);
     inc(c[y]);
   end;
   for i:= to n do
   begin
     for j:= to n do
       f[i,j]:=-;
     pos[i,i]:=i;
     f[i,i]:=;
     bfs(i);
   end;
   writeln(dfs(s,t)::);
 end.