Problem:
Score : 600 points

Problem Statement

On the xy-plane, Snuke is going to travel from the point (x_s, y_s) to the point (x_t, y_t).
He can move in arbitrary directions with speed 1.
Here, we will consider him as a point without size.

There are N circular barriers deployed on the plane.
The center and the radius of the i-th barrier are (x_i, y_i) and r_i, respectively.
The barriers may overlap or contain each other.

A point on the plane is exposed to cosmic rays if the point is not within any of the barriers.

Snuke wants to avoid exposure to cosmic rays as much as possible during the travel.
Find the minimum possible duration of time he is exposed to cosmic rays during the travel.

Constraints

All input values are integers.

-10^9 ≤ x_s, y_s, x_t, y_t ≤ 10^9

(x_s, y_s) ≠ (x_t, y_t)

1≤N≤1,000

-10^9 ≤ x_i, y_i ≤ 10^9

1 ≤ r_i ≤ 10^9

Input

The input is given from Standard Input in the following format:

x_s y_s x_t y_t
N
x_1 y_1 r_1
x_2 y_2 r_2
:
x_N y_N r_N

Output

Print the minimum possible duration of time Snuke is exposed to cosmic rays during the travel.
The output is considered correct if the absolute or relative error is at most 10^{-9}.

Sample Input 1

-2 -2 2 2
1
0 0 1

Sample Output 1

3.6568542495

An optimal route is as follows:

Sample Input 2

-2 0 2 0
2
-1 0 2
1 0 2

Sample Output 2

0.0000000000

An optimal route is as follows:

Sample Input 3

4 -2 -2 4
3
0 0 2
4 0 1
0 4 1

Sample Output 3

4.0000000000

An optimal route is as follows:

Solution in C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define graph_valtype int
#define heap_valtype pair
#define inf (1e18)

typedef struct {
	int v_num;
	double dis;
}pair;

int compare(heap_valtype a, heap_valtype b){
	if(a.dis < b.dis){
		return -1;
	}
	else if(a.dis == b.dis){
		return 0;
	}
	else{
		return 1;
	}
}


typedef struct graph_edge_sub graph_edge;

typedef struct {
	int num;
	int next_num;
	graph_edge *next;
	int prev_num;
}graph_vertex_sub;

struct graph_edge_sub{
	graph_vertex_sub *v;
	double w;
	graph_edge *next;
};

typedef struct graph_v_sub graph_vertex;

struct graph_v_sub{
	int num;
	graph_valtype val;
	int next_num;
	graph_vertex **next;
	double *next_weight;
	int prev_num;
	graph_vertex **prev;
	double *prev_weight;
};

typedef struct {
	int N;
	graph_vertex_sub **v_s;
	graph_vertex **v;
}graph;

//頂点数N, 各頂点の初期値ini_valのグラフを作る
graph *make_graph(int N, graph_valtype ini_val){
	int i;
	graph *g = (graph *)malloc(sizeof(graph));
	g->N = N;
	g->v_s = (graph_vertex_sub **)malloc(sizeof(graph_vertex_sub *) * N);
	g->v = (graph_vertex **)malloc(sizeof(graph_vertex *) * N);
	for(i = 0; i < N; i++){
		(g->v_s)[i] = (graph_vertex_sub *)malloc(sizeof(graph_vertex_sub));
		(g->v_s)[i]->num = i;
		(g->v_s)[i]->next_num = 0;
		(g->v_s)[i]->next = NULL;
		(g->v_s)[i]->prev_num = 0;
		(g->v)[i] = (graph_vertex *)malloc(sizeof(graph_vertex));
		(g->v)[i]->num = i;
		(g->v)[i]->val = ini_val;
		(g->v)[i]->next_num = 0;
		(g->v)[i]->next = NULL;
		(g->v)[i]->next_weight = NULL;
		(g->v)[i]->prev_num = 0;
		(g->v)[i]->prev = NULL;
		(g->v)[i]->prev_weight = NULL;
	}
	return g;
}

//グラフgの頂点aから頂点bに重みwの有向辺を張る (0 <= a, b <= N - 1)
void set_edge_graph(int a, int b, double w, graph *g){
	graph_edge *new1 = (graph_edge *)malloc(sizeof(graph_edge));
	new1->v = (g->v_s)[b];
	new1->w = w;
	new1->next = (g->v_s)[a]->next;
	(g->v_s)[a]->next = new1;
	(g->v_s)[a]->next_num++;
	(g->v_s)[b]->prev_num++;
}

//set_edge_graph後に呼び出す
void build_graph(graph *g){
	int i;
	graph_vertex_sub **v_s = g->v_s;
	graph_vertex **v = g->v;
	graph_vertex *nowv;
	graph_edge *nowe;
	for(i = 0; i < g->N; i++){
		v[i]->next = (graph_vertex **)malloc(sizeof(graph_vertex *) * v_s[i]->next_num);
		v[i]->next_weight = (double *)malloc(sizeof(double) * v_s[i]->next_num);
		v[i]->prev = (graph_vertex **)malloc(sizeof(graph_vertex *) * v_s[i]->prev_num);
		v[i]->prev_weight = (double *)malloc(sizeof(double) * v_s[i]->prev_num);
	}
	for(i = 0; i < g->N; i++){
		nowv = v[i];
		for(nowe = v_s[i]->next; nowe != NULL; nowe = nowe->next){
			(nowv->next)[nowv->next_num] = v[nowe->v->num];
			(nowv->next_weight)[nowv->next_num] = nowe->w;
			nowv->next_num++;
			(v[nowe->v->num]->prev)[v[nowe->v->num]->prev_num] = nowv;
			(v[nowe->v->num]->prev_weight)[v[nowe->v->num]->prev_num] = nowe->w;
			v[nowe->v->num]->prev_num++;
		}
	}
}

typedef struct heap_node_sub{
	heap_valtype val;
	struct heap_node_sub *parent;
	struct heap_node_sub *prev;
	struct heap_node_sub *next;
	struct heap_node_sub *left;
	struct heap_node_sub *right;
}heap_node;

typedef struct {
	int N;
	heap_node *root;
	heap_node *last;
}heap;

//heapを生成
heap *make_heap(){
	heap *h = (heap *)malloc(sizeof(heap));
	h->N = 0;
	h->root = NULL;
	h->last = NULL;
	return h;
}

int element_num_heap(heap *h){
	return h->N;
}

void up_heapify(heap_node *n, heap *h){
	if(n->parent != NULL){
		if(compare(n->val, n->parent->val) < 0){
			heap_valtype tmp;
			tmp = n->val;
			n->val = n->parent->val;
			n->parent->val = tmp;
			up_heapify(n->parent, h);
		}
	}
}

//データを追加
void add_data_heap(heap_valtype val, heap *h){
	heap_node *new = (heap_node *)malloc(sizeof(heap_node));
	new->val = val;
	new->next = NULL;
	new->left = NULL;
	new->right = NULL;
	if(element_num_heap(h) == 0){
		new->parent = NULL;
		new->prev = NULL;
		h->root = new;
		h->last = new;
	}
	else if(element_num_heap(h) == 1){
		new->parent = h->root;
		new->prev = h->root;
		h->root->left = new;
		h->root->next = new;
		h->last = new;
		up_heapify(new, h);
	}
	else{
		new->prev = h->last;
		h->last->next = new;
		if(h->last->parent->right == NULL){
			new->parent = h->last->parent;
			h->last->parent->right = new;
		}
		else{
			new->parent = h->last->parent->next;
			h->last->parent->next->left = new;
		}
		h->last = new;
		up_heapify(new, h);
	}
	h->N++;
}

void down_heappify(heap_node *n, heap *h){
	if(n->left != NULL){
		heap_valtype tmp;
		if(n->right == NULL){
			if(compare(n->val, n->left->val) > 0){
				tmp = n->val;
				n->val = n->left->val;
				n->left->val = tmp;
				down_heappify(n->left, h);
			}
		}
		else{
			if(compare(n->left->val, n->right->val) <= 0 && compare(n->val, n->left->val) > 0){
				tmp = n->val;
				n->val = n->left->val;
				n->left->val = tmp;
				down_heappify(n->left, h);
			}
			else if(compare(n->left->val, n->right->val) > 0 && compare(n->val, n->right->val) > 0){
				tmp = n->val;
				n->val = n->right->val;
				n->right->val = tmp;
				down_heappify(n->right, h);
			}
		}
	}
}

//最小のものを取り出す
heap_valtype take_min_heap(heap *h){
	if(element_num_heap(h) == 0){
		printf("no data in the heap\n");
	}
	heap_valtype ans = h->root->val;
	heap_node *ln = h->last;
	if(element_num_heap(h) == 1){
		h->root = NULL;
		h->last = NULL;
	}
	else{
		ln->prev->next = NULL;
		if(ln->parent->right == NULL){
			ln->parent->left = NULL;
		}
		else{
			ln->parent->right = NULL;
		}
		h->last = ln->prev;
		h->root->val = ln->val;
		down_heappify(h->root, h);
	}
	free(ln);
	h->N--;
	return ans;
}

heap_valtype look_min_heap(heap *h){
	if(element_num_heap(h) == 0){
		printf("no data in the heap\n");
	}
	return h->root->val;
}

double *Dijkstra(graph *g, int s){
	int N = g->N, i;
	double *distance = (double *)malloc(sizeof(double) * N);
	for(i = 0; i < N; i++){
		distance[i] = inf;
	}
	heap *h = make_heap();
	pair p;
	p.v_num = s;
	p.dis = 0;
	add_data_heap(p, h);
	graph_vertex *nowv;
	while(element_num_heap(h) > 0){
		p = take_min_heap(h);
		nowv = g->v[p.v_num];
		if(p.dis < distance[nowv->num]){
			distance[nowv->num] = p.dis;
			for(i = 0; i < nowv->next_num; i++){
				p.v_num = nowv->next[i]->num;
				p.dis = distance[nowv->num] + nowv->next_weight[i];
				add_data_heap(p, h);
			}
		}
	}
	return distance;
}

double max(double a, double b){
	return a >= b ? a : b;
}

int main(){
	int N, i, j;
	double xs, ys, xt, yt;
	scanf("%lf%lf%lf%lf", &xs, &ys, &xt, &yt);
	scanf("%d", &N);
	double *x = (double *)malloc(sizeof(double) * (N + 2));
	double *y = (double *)malloc(sizeof(double) * (N + 2));
	double *r = (double *)malloc(sizeof(double) * (N + 2));
	x[0] = xs;
	y[0] = ys;
	r[0] = 0;
	for(i = 1; i <= N; i++){
		scanf("%lf%lf%lf", &x[i], &y[i], &r[i]);
	}
	x[N + 1] = xt;
	y[N + 1] = yt;
	r[N + 1] = 0;
	graph *g = make_graph(N + 2, 0);
	for(i = 0; i <= N + 1; i++){
		for(j = 0; j <= N + 1; j++){
			if(i != j){
				set_edge_graph(i, j, max(hypot(x[i] - x[j], y[i] - y[j]) - (r[i] + r[j]), 0), g);
			}
		}
	}
	build_graph(g);
	printf("%.12lf\n", Dijkstra(g, 0)[N + 1]);
	return 0;
}