SGU438_The Glorious Karlutka River =)

#include <iostream>
#include <cstdio>
#include <cstring>
#define maxn 55555
#define maxm 9999999
using namespace std;

int to[maxm],next[maxm],c[maxm],first[maxn],edge;
int Q[maxm],bot,top,node;
int tag[maxn],d[maxn],TAG=520;
int L[55],R[55];
bool can[maxn],iq[maxn];
int X[55],Y[55],C[55],connect[55][55];
int n,m,D,W,s,t,ans;

int addnode()
{
    first[++node]=-1;
    return node;
}

void addedge(int U,int V,int W)
{
    edge++;
    to[edge]=V,c[edge]=W,next[edge]=first[U],first[U]=edge;
    edge++;
    to[edge]=U,c[edge]=0,next[edge]=first[V],first[V]=edge;
}

bool _input()
{
    bot=1,top=0;
    scanf("%d%d%d%d",&n,&m,&D,&W);
    for (int i=1; i<=n; i++)
    {
        iq[i]=false;
        scanf("%d%d%d",&X[i],&Y[i],&C[i]);
        if (C[i]==0) 
        {
            i--,n--;
            continue;
        }
        if (Y[i]<=D) Q[++top]=i,iq[i]=true;
    }
    memset(connect,false,sizeof connect);
    for (int i=1; i<=n; i++)
        for (int j=1; j<=n; j++)
            if (i!=j && (X[i]-X[j])*(X[i]-X[j])+(Y[i]-Y[j])*(Y[i]-Y[j])<=D*D)
                connect[i][j]=connect[j][i]=true;
    while (bot<=top)
    {
        int cur=Q[bot++];
        if (Y[cur]+D>=W) return true;
        for (int i=1; i<=n; i++)
            if (connect[cur][i] && !iq[i]) Q[++top]=i,iq[i]=true;
    }
    if (D<W) return false;
        else return true;
}

void build_init_graph()
{
    edge=-1,node=0;
    s=addnode(),t=addnode();
    for (int i=1; i<=n; i++) L[i]=addnode(),R[i]=addnode();
    for (int i=1; i<=n; i++)
    {
        addedge(L[i],R[i],C[i]);
        if (Y[i]<=D) addedge(s,L[i],C[i]);
        if (Y[i]+D>=W) addedge(R[i],t,C[i]);
    }
}

bool bfs()
{
    Q[bot=top=1]=t,d[t]=0,tag[t]=++TAG,can[t]=false;
    while (bot<=top)
    {
        int cur=Q[bot++];
        for (int i=first[cur]; i!=-1; i=next[i])
            if (c[i^1]>0 && tag[to[i]]!=TAG)
            {
                tag[to[i]]=TAG,d[to[i]]=d[cur]+1;
                can[to[i]]=false,Q[++top]=to[i];
                if (to[i]==s) return true;
            }
    }
    return false;
}

int dfs(int cur,int num)
{
    if (cur==t) return num;
    int tmp=num,k;
    for (int i=first[cur]; i!=-1; i=next[i])
        if (c[i]>0 && d[to[i]]==d[cur]-1 && tag[to[i]]==TAG && !can[to[i]])
        {
            k=dfs(to[i],min(num,c[i]));
            if (k) num-=k,c[i]-=k,c[i^1]+=k;
            if (num==0) break;
        }
    if (num) can[cur]=true;
    return tmp-num;
}

int maxflow()
{
    int tot=0;
    while (bfs()) tot+=dfs(s,maxm);
    return tot;
}

int main()
{
    if (!_input()) { puts("IMPOSSIBLE"); return 0; }
    if (D>=W) { puts("1"); return 0; }
    build_init_graph();
    for (ans=2; maxflow()<m; ans++)
    {
        for (int i=1; i<=edge; i+=2) if (c[i]>0) c[i-1]+=c[i],c[i]=0;
        for (int i=1; i<=n; i++)
        {
            L[i]=addnode();
            if (Y[i]<=D) addedge(s,L[i],C[i]);
            for (int j=1; j<=n; j++)
                if (connect[i][j]) addedge(R[j],L[i],C[i]);
        }
        for (int i=1; i<=n; i++)
        {
            R[i]=addnode();
            addedge(L[i],R[i],C[i]);
            if (Y[i]+D>=W) addedge(R[i],t,C[i]);
        }
    }
    printf("%d\n",ans);
    return 0;
}