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
}

type Memo struct {
	NowPoint             int
	DiscountTicketTCount 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, n)
		}
		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, m)
		start := Node{NowPosition: s, DiscountTicketTCount: c, Amount: 0}
		heap.Push(&pq, &start)
		memo := make(map[Memo]int)
		for len(pq) != 0 {
			now := heap.Pop(&pq).(*Node)
			if now.NowPosition == d {
				fmt.Println(now.Amount)
				break
			}
			memo[Memo{NowPoint: now.NowPosition, DiscountTicketTCount: now.DiscountTicketTCount}] = now.Amount
			for _, v := range pointMap[now.NowPosition] {
				next := &Node{v.NextPoint, now.DiscountTicketTCount, now.Amount + v.Fee}
				if memoAmount, ok := memo[Memo{NowPoint: v.NextPoint, DiscountTicketTCount: now.DiscountTicketTCount}]; !ok || (ok && memoAmount > next.Amount) {
					memo[Memo{NowPoint: v.NextPoint, DiscountTicketTCount: now.DiscountTicketTCount}] = next.Amount
					heap.Push(&pq, next)
				}
				if now.DiscountTicketTCount > 0 {
					next = &Node{v.NextPoint, now.DiscountTicketTCount - 1, now.Amount + v.Fee/2}
					if memoAmount, ok := memo[Memo{NowPoint: v.NextPoint, DiscountTicketTCount: now.DiscountTicketTCount}]; !ok || (ok && memoAmount > next.Amount) {
						memo[Memo{NowPoint: v.NextPoint, DiscountTicketTCount: now.DiscountTicketTCount}] = next.Amount
						heap.Push(&pq, next)
					}
				}
			}
		}
		//runtime.GC()
	}
}

C++: #include <iostream>
#include <vector>
#include <queue>
using namespace std;

struct state {
  int node;
  int ticket;
  int cost;
  state(int n, int t, int c) : node(n), ticket(t), cost(c) {}
  bool operator<(const state& s) const
  {
    return cost > s.cost;
  }
};

int main()
{
  int c, n, m, s, d;
  while (cin >> c >> n >> m >> s >> d && c != 0) {
    --s;  --d;
    vector<vector<pair<int,int> > > g(n);
    for (int i = 0; i < m; i++) {
      int a, b, f;
      cin >> a >> b >> f;
      --a;  --b;
      g[a].push_back(make_pair(b, f));
      g[b].push_back(make_pair(a, f));
    }

    priority_queue<state> q;
    q.push(state(s, c, 0));
    vector<vector<int> > city(n, vector<int>(c, 1000000));
    city[s][c] = 0;
    while (!q.empty()) {
      const state s = q.top();
      q.pop();
      if (s.node == d) {
        cout << s.cost << endl;
        break;
      }

      city[s.node][s.ticket] = s.cost;
      for (vector<pair<int,int> >::const_iterator it(g[s.node].begin()); it != g[s.node].end(); ++it) {
        const int next = it->first;
        const int cost = s.cost + it->second;
        if (cost < city[next][s.ticket]) {
          city[next][s.ticket] = cost;
          q.push(state(next, s.ticket, cost));
        }
        if (s.ticket > 0) {
          const int cost2 = s.cost + it->second/2;
          if (cost2 < city[next][s.ticket-1]) {
            city[next][s.ticket-1] = cost2;
            q.push(state(next, s.ticket-1, cost2));
          }
        }
      }
    }
  }
  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>

using namespace std;

#define rep(i,j) REP((i), 0, (j))
#define REP(i,j,k) for(int i=(j);(i)<(k);++i)
#define BW(a,x,b) ((a)<=(x)&&(x)<=(b))
#define ALL(v) (v).begin(), (v).end()
#define LENGTHOF(x) (sizeof(x) / sizeof(*(x)))
#define AFILL(a, b) fill((int*)a, (int*)(a + LENGTHOF(a)), b)
#define MP make_pair
#define PB push_back
#define F first
#define S second
#define INF 1 << 30
#define EPS 1e-10

typedef pair<int, int> pi; // 残りの割引券 現在の街
typedef pair<int, pi>P; // コスト pi
typedef vector<int> vi;
typedef queue<int> qi;
typedef long long ll;

struct edge{ int to, cost; };

int C, V, E, S, D;
int d[128][16];
 vector<vector<edge> >G(128);
// int G[128];

void dijkstra(int s){
  priority_queue<P, vector<P>, greater<P> >q;
  rep(i, V) rep(j, C+1) d[i][j] = INF;
  d[s][C] = 0;
  q.push(MP(0, MP(C, s)));

  while(!q.empty()){
    P p = q.top(); q.pop();
    int v = p.S.S;
    int c = p.S.F;

    rep(i, G[v].size()){
      edge e = G[v][i];
      if(d[e.to][c] > d[v][c] + e.cost){
	d[e.to][c] = d[v][c] + e.cost;
	q.push(MP(d[e.to][c], MP(c, e.to)));
      }

      if( c > 0 && d[e.to][c-1] > d[v][c] + e.cost/2){
	  d[e.to][c-1] = d[v][c] + e.cost/2;
	  q.push(MP(d[e.to][c-1], MP(c-1, e.to)));
      }
    }
  }
}

int main(){
  while(scanf("%d%d%d%d%d", &C, &V, &E, &S, &D) && C+V+E+S+D){
    S--; D--;
    rep(i, 128){
      //      rep(j, 128) G[i][j] = INF;
      G[i].clear();
    }
    rep(i, E){
      int a, b, c;
      scanf("%d%d%d", &a, &b, &c); a--; b--;
     edge e;
      e.to = a; e.cost = c;
      G[b].PB(e);
      e.to = b;
      G[a].PB(e);
      //      G[a][b] = G[b][a] = c;
    }

    dijkstra(S);

    int res = INF;
    rep(i, C+1) res = min(res, d[D][i]);
    printf("%d\n", res);
    //    printf("%d\n", d[D][0]);
  }

  return 0;
}