SGU 438 -- The Glorious Karlutka River =)

#include<cstdio>
#include<cstring>
#include<queue>
#include<iostream>
#include<vector>
#include<cmath>
#define Min(a,b) ((a)<(b)?(a):(b))
#define Max(a,b) ((a)>(b)?(a):(b))
#define P(a, b) ((a-1)*50+(b))
#define Mac(a) ((a)+5000)
using namespace std;
const int N=10005, M=1210010, INF=0x3f3f3f3f;
int n, m, d, w, s, t, top;
int head
, gap
, cur
, dis
, vis
, pre
, g[55][55];

struct Point{
int x, y, cap;
}p[55];

struct Edge{
int to, next, flow;
Edge(int _to = 0, int _next = 0, int _flow = 0):to(_to), next(_next), flow(_flow){}
}edge[M];

bool Judge(int a, int b){
double x = d, y = sqrt(double((p[a].x-p[b].x)*(p[a].x-p[b].x)+(p[a].y-p[b].y)*(p[a].y-p[b].y)));
if(x - y >= -1e-6) return 1;
return 0;
}

void Addedge(int from, int to, int flow){
edge[top] = Edge(to, head[from], flow);
head[from] = top++;
edge[top] = Edge(from, head[to], 0);
head[to] = top++;
}

void Bfs(){
queue<int> q;
memset(gap, 0, sizeof(gap));
memset(dis, -1, sizeof(dis));
gap[0] = 1; dis[t] = 0; q.push(t);
while(!q.empty()){
int u = q.front(); q.pop();
for(int i = head[u]; i+1; i = edge[i].next){
if(dis[edge[i].to] == -1){
dis[edge[i].to] = dis[u] + 1;
gap[dis[edge[i].to]] ++;
q.push(edge[i].to);
}
}
}
}

int Sap(){
Bfs();
memset(pre, -1, sizeof(pre));
for(int i = s; i <= t; ++i) cur[i] = head[i];
int u = s, cur_flow, max_flow = 0, neck, tmp;
while(dis[s] <= t){
if(u == t){
cur_flow = INF;
for(int i = s; i != t; i = edge[cur[i]].to){
if(cur_flow > edge[cur[i]].flow){
cur_flow = edge[cur[i]].flow;
neck = i;
}
}
for(int i = s; i != t; i = edge[cur[i]].to){
tmp = cur[i];
edge[tmp].flow -= cur_flow;
edge[tmp^1].flow += cur_flow;
}
max_flow += cur_flow;
u = neck;
}
int i;
for(i = cur[u]; i + 1; i = edge[i].next){
if(edge[i].flow && dis[u] == dis[edge[i].to] + 1) break;
}
if(i != -1){
cur[u] = i;
pre[edge[i].to] = u;
u = edge[i].to;
}else{
if(--gap[dis[u]] == 0) break;
cur[u] = head[u];
int mindis = t;
for(i = head[u]; i + 1; i = edge[i].next)
if(edge[i].flow && mindis > dis[edge[i].to]) mindis = dis[edge[i].to];
dis[u] = mindis + 1;
gap[dis[u]] ++;
if(u != s) u = pre[u];
}
}
return max_flow;
}

int main(){
while(~scanf("%d%d%d%d", &n, &m, &d, &w)){
memset(head, -1, sizeof(head));
memset(g, 0, sizeof(g));
top = s = 0; t = 10001;
int ans = 0, res, flag = 0;
for(int i = 1; i <= n; ++i) scanf("%d%d%d", &p[i].x, &p[i].y, &p[i].cap);
if(d >= w){
printf("1\n"); continue;
}
for(int i = 1; i <= n; ++i)
for(int j = 1; j <= n; ++j)
if(i != j && Judge(i, j)) g[i][j] = 1;
for(int i = 1; i <= n+m; ++i){
for(int j = 1; j <= n; ++j){
if(p[j].y <= d) Addedge(s, P(i, j), INF);
if(p[j].y+d >= w) Addedge(Mac(P(i, j)), t, INF);
Addedge(P(i, j), Mac(P(i, j)), p[j].cap);
for(int k = 1; k <= n; ++k){
if(k != j && g[j][k] == 1) Addedge(Mac(P(i, j)), P(i+1, k), INF);
}
}
res = Sap();
ans += res;
if(ans >= m){
flag = 1;
printf("%d\n", i + 1);
break;
}
}
if(!flag) printf("IMPOSSIBLE\n");
}
}