学步园

//author: CHC
//First Edit Time:	2014-10-16 10:05
//Last Edit Time:	2014-10-16 11:16
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <set>
#include <vector>
#include <map>
#include <queue>
#include <set>
#include <algorithm>
#include <limits>
using namespace std;
typedef long long LL;
const int MAXN=1e+4;
const int MAXM=1e+5;
const int INF= numeric_limits<int>::max();
const LL LL_INF= numeric_limits<LL>::max();
struct Edge
{
    int from,to,ci,cost,next;
    Edge(){}
    Edge(int _from,int _to,int _ci,int _cost,int _next):from(_from),to(_to),ci(_ci),cost(_cost),next(_next){}
}e[MAXM];
int head[MAXN],tot;
int q[MAXM];
int dis[MAXN],pre[MAXN],rec[MAXN],vis[MAXN];
inline void init(){
    memset(head,-1,sizeof(head));
    tot=0;
}
inline void AddEdge1(int u,int v,int ci,int cost){
    e[tot]=Edge(u,v,ci,cost,head[u]);
    head[u]=tot++;
    e[tot]=Edge(v,u,0,-cost,head[v]);
    head[v]=tot++;
}
inline bool spfa(int S,int T,LL &cost,LL &flow){
    int i,h=0,t=0;
    for(i=0;i<=MAXN;i++){
        dis[i]=INF;
        vis[i]=false;
    }
    q[h]=S;
    dis[S]=0;
    vis[S]=true;
    while(h<=t){
        int u=q[h++];
        vis[u]=false;
        for(i=head[u];~i;i=e[i].next){
            int v=e[i].to;
            if(e[i].ci>0&&dis[v]>dis[u]+e[i].cost){
                dis[v]=dis[u]+e[i].cost;
                pre[v]=u;
                rec[v]=i;
                if(!vis[v]){
                    vis[v]=1;
                    q[++t]=v;
                }
            }
        }
    }
    if(dis[T]==INF)return false;
    int minn=INF;
    for(i=T;i!=S;i=pre[i]){
        if(e[rec[i]].ci<minn)
            minn=e[rec[i]].ci;
    }
    for(i=T;i!=S;i=pre[i]){
        //cost+=minn*e[rec[i]].cost;
        e[rec[i]].ci-=minn;
        e[rec[i]^1].ci+=minn;
    }
    cost+=dis[T]*minn;
    flow+=minn;
    return true;
}
inline void mincostmaxflow(int S,int T,LL &cost,LL &flow){
    while(spfa(S,T,cost,flow));
}
int du[MAXN];
int main()
{
    int t,cas=0;
    scanf("%d",&t);
    while(t--){
        int n,m,s,t;
        scanf("%d%d%d%d",&n,&m,&s,&t);
        init();
        memset(du,0,sizeof(du));
        LL sum=0;
        for(int i=0,u,v,a,b;i<m;i++){
            scanf("%d%d%d%d",&u,&v,&a,&b);
            if(b>=a){
                //u->v保留了下来，如果要移除u->v这条边，那么会获得b-a花费
                AddEdge1(v,u,1,b-a),sum+=a;
                ++du[v];--du[u];
            }
            else {
                //u->v没有保存下来，如果要添加u->v这条边，那么会获得a-b花费
                AddEdge1(u,v,1,a-b),sum+=b;
            }
        }
        ++du[s];--du[t];
        LL ss=0;
        //AddEdge1(s,t,1,INF);
        for(int i=1;i<=n;i++){
            if(du[i]>0)AddEdge1(0,i,du[i],0);
            if(du[i]<0)AddEdge1(i,n+1,-du[i],0),ss+=-du[i];
        }
        LL cost=0,flow=0;
        mincostmaxflow(0,n+1,cost,flow);
        printf("Case %d: ",++cas);
        if(flow!=ss)puts("impossible");
        //printf("%I64d\n",sum);
        else
        printf("%I64d\n",sum+cost);
    }
    scanf(" ");
    return 0;
}