加从t到s的流量无穷大，然后在可行流的残留网络上跑最大流。
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int maxn = 210,maxm = 20800,inf = 1000000000;
int cnt = 1,ss,tt,s,t,n,m;
int head[maxn],dis[maxn],d[maxn],nxt[maxm],to[maxm],flow[maxm],low[maxm];
bool inq[maxn];
void add(int a,int b,int fl)
{
nxt[++cnt] = head[a];
to[cnt] = b;
head[a] = cnt;
flow[cnt] = fl;
nxt[++cnt] = head[b];
to[cnt] = a;
head[b] = cnt;
flow[cnt] = 0;
}
bool bfs()
{
queue <int> que;
memset(inq,0,sizeof(inq));
que.push(tt);
inq[tt] = 1;
while (!que.empty())
{
int cur = que.front();
que.pop();
for (int i = head[cur];i;i = nxt[i])
{
int v = to[i];
if (!inq[v] && flow[i ^ 1])
{
dis[v] = dis[cur] + 1;
que.push(v);
inq[v] = 1;
}
}
}
return inq[ss];
}
int dfs(int cur,int lmt)
{
if (cur == tt) return lmt;
int fl = 0;
for (int i = head[cur];i && fl < lmt;i = nxt[i])
{
if (dis[to[i]] + 1 == dis[cur] && flow[i])
{
int tt = dfs(to[i],min(lmt - fl,flow[i]));
flow[i] -= tt;
flow[i ^ 1] += tt;
fl += tt;
}
}
return fl;
}
int maxflow()
{
int res = 0;
while (bfs())
{
int fl = 0;
do
{
fl = dfs(ss,inf);
res += fl;
} while (fl);
}
return res;
}
//cnt = 1
int main()
{
scanf("%d%d%d%d",&n,&m,&s,&t);
ss = n + 1;
tt = n + 2;
int tx,ty,tl,tu;
for (int i = 1;i <= m;i++)
{
scanf("%d%d%d%d",&tx,&ty,&tl,&tu);
add(tx,ty,tu - tl);
d[tx] -= tl;
d[ty] += tl;
low[i] = tl;
}
add(t,s,inf);
int sum = 0;
for (int i = 1;i <= n;i++)
if (d[i] > 0)
{
sum += d[i];
add(ss,i,d[i]);
}else if (d[i] < 0)
add(i,tt,-d[i]);
if (maxflow() != sum)
{
printf("please go home to sleep\n");
return 0;
}else
{
ss = s;
tt = t;
printf("%d\n",maxflow());
}
return 0;
}