Go: package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
	"strings"
)

var scanner = bufio.NewScanner(os.Stdin)

const INF = 0x7FFFFFFF

var pass [][]int

func main() {
	for scanner.Scan() {
		pass = make([][]int, 200)
		for i := range pass {
			pass[i] = make([]int, 200)
		}
		buf := strings.Split(scanner.Text(), " ")
		c, _ := strconv.Atoi(buf[0])
		n, _ := strconv.Atoi(buf[1])
		m, _ := strconv.Atoi(buf[2])
		s, _ := strconv.Atoi(buf[3])
		d, _ := strconv.Atoi(buf[4])
		if c == 0 && n == 0 && m == 0 && s == 0 && d == 0 {
			break
		}
		s--
		d--
		for i := 0; i < 200; i++ {
			for j := 0; j < 200; j++ {
				pass[i][j] = INF
			}
		}
		for i := 0; i < 200; i++ {
			pass[i][i] = 0
		}
		for i := 0; i < m; i++ {
			scanner.Scan()
			buf_ := strings.Split(scanner.Text(), " ")
			a, _ := strconv.Atoi(buf_[0])
			b, _ := strconv.Atoi(buf_[1])
			f, _ := strconv.Atoi(buf_[2])
			a--
			b--
			pass[a][b] = f
			pass[b][a] = f
		}
		fmt.Println(dijkstra(s, d, c, n))
	}
}

func dijkstra(s, g, c, n int) float64 {
	d := make([][]float64, 200)
	used := make([][]bool, 200)
	for i := 0; i < 200; i++ {
		d[i] = make([]float64, 20)
		used[i] = make([]bool, 20)
	}
	for i := range d {
		for j := range d[i] {
			d[i][j] = INF
		}
	}
	d[s][c] = 0
	for {
		v := -1
		w := -1
		for u := 0; u < n; u++ {
			for i := 0; i < c+1; i++ {
				if !used[u][i] && (v == -1 || d[u][i] < d[v][w]) {
					v = u
					w = i
				}
			}
		}
		if v == -1 {
			break
		}
		used[v][w] = true
		for u := 0; u < n; u++ {
			d[u][w] = math.Min(d[u][w], d[v][w]+float64(pass[v][u]))
			if w-1 >= 0 && pass[v][u] != INF {
				d[u][w-1] = math.Min(d[u][w-1], d[v][w]+(float64(pass[v][u]/2.0)))
			}
		}
	}
	ret := float64(INF)
	for i := 0; i < c+1; i++ {
		ret = math.Min(ret, d[g][i])
	}
	return ret
}

C++: #include <stdio.h>
#include <string.h>
#include <algorithm>
#include <iostream>
#include <math.h>
#include <assert.h>
#include <vector>
#include <queue>

using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
static const double EPS = 1e-9;
static const double PI = acos(-1.0);

#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define FOR(i, s, n) for (int i = (s); i < (int)(n); i++)
#define FOREQ(i, s, n) for (int i = (s); i <= (int)(n); i++)
#define FORIT(it, c) for (__typeof((c).begin())it = (c).begin(); it != (c).end(); it++)
#define MEMSET(v, h) memset((v), h, sizeof(v))

typedef int Weight;
struct Edge {
  int cnt;
  int src;
  int dest;
  Weight weight;
  Edge(int cnt, int src, int dest, Weight weight) : cnt(cnt), src(src), dest(dest), weight(weight) {;}
  bool operator<(const Edge &rhs) const {
    if (weight != rhs.weight) { return weight > rhs.weight; }
    return false;
  }
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;
typedef vector<Weight> Array;
typedef vector<Array> Matrix;

void PrintMatrix(const Matrix &matrix) {
  for (int y = 0; y < (int)matrix.size(); y++) {
    for (int x = 0; x < (int)matrix[y].size(); x++) {
      printf("%d ", matrix[y][x]);
    }
    puts("");
  }
}

int c, n, m, s, d;
bool visit[20][110];
int dist[20][110];

int main() {
  while (scanf("%d %d %d %d %d", &c, &n, &m, &s, &d), c|n|m|s|d) {
    MEMSET(visit, false);
    MEMSET(dist, 0x0f);
    s--; d--;
    Graph g(n);
    REP(i, m) {
      int f, t, cost;
      scanf("%d %d %d", &f, &t, &cost);
      f--; t--;
      g[f].push_back(Edge(0, f, t, cost));
      g[t].push_back(Edge(0, t, f, cost));
    }
    priority_queue<Edge> que;
    que.push(Edge(c, 0xdeadbeaf, s, 0));
    while (!que.empty()) {
      Edge e = que.top();
      que.pop();
      if (visit[e.cnt][e.dest]) { continue; }
      visit[e.cnt][e.dest] = true;
      if (e.dest == d) {
        printf("%d\n", e.weight);
        break;
      }
      FORIT(it, g[e.dest]) {
        if (e.cnt != 0) {
          int ncost = e.weight + it->weight / 2;
          if (ncost < dist[e.cnt - 1][it->dest]) {
            //dist[e.cnt - 1][it->dest] = ncost;
            que.push(Edge(e.cnt - 1, 0xdeadbeaf, it->dest, ncost));
          }
        }
        int ncost = e.weight + it->weight;
        if (ncost < dist[e.cnt - 1][it->dest]) {
          //dist[e.cnt][it->dest] = ncost;
          que.push(Edge(e.cnt, 0xdeadbeaf, it->dest, ncost));
        }
      }
    }
  }
}

C++: // 2010/12/29 Tazoe

#include <iostream>
using namespace std;

int main()
{
	while(true){
		int c, n, m, s, d;
		cin >> c >> n >> m >> s >> d;
		if(c==0&&n==0&&m==0&&s==0&&d==0)
			break;

		int wgt[102][102];
		int hlf[102][102];
		for(int i=1; i<=n; i++)
			for(int j=1; j<=n; j++)
				if(i==j){
					wgt[i][j] = 0;
					hlf[i][j] = 20000;
				}
				else{
					wgt[i][j] = 20000;
					hlf[i][j] = 20000;
				}

		for(int i=0; i<m; i++){
			int a, b, f;
			cin >> a >> b >> f;
			wgt[a][b] = f;
			wgt[b][a] = f;
			hlf[a][b] = f/2;
			hlf[b][a] = f/2;
		}

		int djk[12][102];
		bool flg[12][102];
		for(int j=c; j>=0; j--)
			for(int i=1; i<=n; i++){
				djk[j][i] = 20000;
				flg[j][i] = false;
			}
		djk[c][s] = 0;

		while(true){
			int j_min = 11;
			int i_min = 0;
			int w_min = 2000000;
			for(int j=0; j<=c; j++)
				for(int i=1; i<=n; i++)
					if((!flg[j][i])&&(djk[j][i]<w_min)){
						j_min = j;
						i_min = i;
						w_min = djk[j][i];
					}

			flg[j_min][i_min] = true;
			if(i_min==d){
				cout << djk[j_min][i_min] << endl;
				break;
			}

			for(int i=1; i<=n; i++)
				if(!flg[j_min][i])
					if(wgt[i_min][i]!=20000)
						if(djk[j_min][i]>djk[j_min][i_min]+wgt[i_min][i])
							djk[j_min][i] = djk[j_min][i_min]+wgt[i_min][i];
			if(j_min!=0)
				for(int i=1; i<=n; i++)
					if(!flg[j_min-1][i])
						if(hlf[i_min][i]!=20000)
							if(djk[j_min-1][i]>djk[j_min][i_min]+hlf[i_min][i])
								djk[j_min-1][i] = djk[j_min][i_min]+hlf[i_min][i];
		}
	}

	return 0;
}