Go: package main

import (
	"container/heap"
	"fmt"
)

type Node struct {
	NowPosition          int
	DiscountTicketTCount int
	Amount               int
}

type PriorityQueueNode []*Node

func (pq PriorityQueueNode) Len() int {
	return len(pq)
}

func (pq PriorityQueueNode) Less(i, j int) bool {
	return pq[i].Amount < pq[j].Amount
}

func (pq PriorityQueueNode) Swap(i, j int) {
	pq[i], pq[j] = pq[j], pq[i]
}

func (pq *PriorityQueueNode) Push(node interface{}) {
	*pq = append(*pq, node.(*Node))
}

func (pq *PriorityQueueNode) Pop() interface{} {
	old := *pq
	n := len(old)
	item := old[n-1]
	*pq = old[0 : n-1]
	return item
}

type Point struct {
	NextPoint int
	Fee       int
}

func main() {
	var c, n, m, s, d int
	var a, b, f int

	for {
		fmt.Scan(&c, &n, &m, &s, &d)
		if c == 0 && n == 0 && m == 0 && s == 0 && d == 0 {
			break
		}
		s--
		d--
		pointMap := make([][]Point, n)
		for i := 0; i < n; i++ {
			pointMap[i] = make([]Point, 0)
		}
		for i := 0; i < m; i++ {
			fmt.Scan(&a, &b, &f)
			a--
			b--
			pointMap[a] = append(pointMap[a], Point{NextPoint: b, Fee: f})
			pointMap[b] = append(pointMap[b], Point{NextPoint: a, Fee: f})
		}
		pq := make(PriorityQueueNode, 0)
		start := Node{NowPosition: s, DiscountTicketTCount: c, Amount: 0}
		heap.Push(&pq, &start)
		for len(pq) != 0 {
			now := heap.Pop(&pq).(*Node)
			if now.NowPosition == d {
				fmt.Println(now.Amount)
				break
			}
			for _, v := range pointMap[now.NowPosition] {
				next := &Node{v.NextPoint, now.DiscountTicketTCount, now.Amount + v.Fee}
				heap.Push(&pq, next)
				if now.DiscountTicketTCount > 0 {
					next = &Node{v.NextPoint, now.DiscountTicketTCount - 1, now.Amount + v.Fee/2}
					heap.Push(&pq, next)
				}
			}
		}
	}
}

C++: #include<iostream>
#include<queue>
#include<vector>

using namespace std;

#define REP(i, n) for(int i=0; i<n; i++)

struct edge{
    int to, cost;
};

typedef pair<int, pair<int, int> > pii; // distance, edge, ticket

int INF = 10e8;
int c, n, m, s, d;

int dis[100][11];

int main(){
    while(1){
        cin >> c >> n >> m >> s >> d;

        if(n==0) break;

        vector<edge> g[100];
        REP(i, m){
            int a, b, f;
            cin >> a >> b >> f;
            edge ae;
            ae.to = b;
            ae.cost = f;
            g[a].push_back(ae);
            edge be;
            be.to = a;
            be.cost = f;
            g[b].push_back(be);
        }

        priority_queue<pii, vector<pii>, greater<pii> > que;
        REP(i, n+1){
            REP(j, c+1){
                dis[i][j] = INF;
            }
        }
        dis[s][c] = 0;
        pii init = make_pair(0, make_pair(s, c));
        que.push(init);

        while(!que.empty()){
            pii p = que.top();
            que.pop();

            int v = p.second.first;
            int t = p.second.second;
            if(dis[v][t]<p.first) continue;
            REP(i, g[v].size()){
                edge e = g[v][i];
                if(dis[e.to][t] > dis[v][t] + e.cost){
                    dis[e.to][t] = dis[v][t] + e.cost;
                    que.push(pii(make_pair(dis[e.to][t], make_pair(e.to, t))));
                }
                if(t>0 && dis[e.to][t-1] > dis[v][t] + e.cost / 2){
                    dis[e.to][t-1] = dis[v][t] + e.cost / 2;
                    que.push(pii(make_pair(dis[e.to][t-1], make_pair(e.to, t-1))));
                }
            }
        }
        /*
        REP(i, n+1){
            REP(j, c+1){
                cout << dis[i][j] << " ";
            }
            cout << endl;
        }
        */
        cout << dis[d][0] << endl;
    }
}

C++: #include <iostream>
#include <queue>
#include <vector>

using namespace std;

struct P {
	int from, cost, coupon;

	bool operator >(const P& p) const {
		return cost > p.cost;
	}
};

int ecost[101][101];
int ccost[101][11]; // 0:not use coupon 1:use coupon
int c, n, m, s, d;
int i;

int main() {
	while(cin >> c >> n >> m >> s >> d, c|n|m|s|d) {
		fill_n((int *)ecost, 101*101, -1);
		fill_n((int *)ccost, 101*11, 1e8);
		for(i=0; i<m; i++){
			int a, b, f;
			cin >> a >> b >> f;
			ecost[a][b] = f;
			ecost[b][a] = f;
		}
		priority_queue<P, vector<P>, greater<P>> q;
		q.push((P){s, 0, 0});
		int dcost;
		while(!q.empty()){
			P p = q.top(); q.pop();
			// goal
			if(p.from == d) {
				dcost = p.cost;
				break;
			}
			//
			for(i=0; i<n+1; i++){ // i:to
				if(ecost[p.from][i] != -1 && p.cost + ecost[p.from][i] < ccost[i][p.coupon]){
					q.push((P){i, p.cost + ecost[p.from][i], p.coupon});
					if(p.coupon+1 <= c) {
						if(p.cost + ecost[p.from][i]/2 < ccost[i][p.coupon+1]){
							q.push((P){i, p.cost + ecost[p.from][i]/2, p.coupon+1});
						}
					}
					//ecost[p.from][i] = -1;
					//ecost[i][p.from] = -1;
				}
			}
		}
		cout << dcost << endl;
	}
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
typedef pair<int, int> pii;
typedef pair<int, pii> piii;//cost, v, ticket
struct edge{
  int to, cost;
};

int main(int argc, char *argv[]){
  int c, n, m, s, d;
  while(std::cin >> c >> n >> m >> s >> d, n){
    s--;d--;
    vector<vector<edge> > g(n);
    for (int i = 0; i < m; i++) {
      edge e;
      int from;
      std::cin >> from >> e.to >> e.cost;
      e.to--;from--;
      g[from].push_back(e);
      g[e.to].push_back((edge){from, e.cost});
    }
    vector<vector<int> > dp(c + 1, vector<int>(n, 1e9));
    dp[0][s] = 0;
    priority_queue<piii, vector<piii>, greater<piii> > que;
    que.push(piii(0, pii(s, 0)));
    while(!que.empty()){
      int cost   = que.top().first;
      int v      = que.top().second.first;
      int ticket = que.top().second.second;
      que.pop();
      for (int i = 0; i < g[v].size(); i++) {
        edge e = g[v][i];
        if(dp[ticket][e.to] > dp[ticket][v] + e.cost){
          dp[ticket][e.to] = dp[ticket][v] + e.cost;
          que.push(piii(dp[ticket][e.to], pii(e.to, ticket)));
        }
        if(ticket + 1 <= c &&
           dp[ticket + 1][e.to] > dp[ticket][v] + e.cost/2){
          dp[ticket + 1][e.to] = dp[ticket][v] + e.cost/2;
          que.push(piii(dp[ticket + 1][e.to],
                        pii(e.to, ticket + 1)));
        }
      }
    }
    int ans = 1e9;
    for (int i = 0; i <= c; i++) {
      ans = min(ans, dp[i][d]);
    }
    std::cout << ans << std::endl;
  }
  return 0;
}