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<utility>
#include<queue>
#include<algorithm>
using namespace std;
#define INF 1000000000
int C,N,M,S,D,g1[101][101],g2[101][101],s,t,c,d[10][101];
typedef struct P{int p,c,t;}P;
struct Order
{
	bool operator ()(P const& a, P const& b) const
	{
		return a.c==b.c?a.t<b.t:a.c>b.c;
	}
};

P tmp,tmp2;
priority_queue<P,vector<P>,Order>Q;
int main()
{
	for(;scanf("%d%d%d%d%d",&C,&N,&M,&S,&D),C;)
	{
		for(int i=0;i<=N;i++)
			for(int j=0;j<=N;j++)
				g1[i][j]=g2[i][j]=INF;
		for(int i=0;i<=C;i++)
			for(int j=0;j<=N;j++)
				d[i][j]=INF;
		for(int i=0;i<M;i++)
		{
			scanf("%d%d%d",&s,&t,&c);
			if(g1[s][t]>c)
			{
				g1[s][t]=g1[t][s]=c;
				g2[s][t]=g2[t][s]=c/2;
			}
		}
		d[C][S]=0;
		tmp.c=0;tmp.p=S;tmp.t=C;
		Q.push(tmp);
		for(;!Q.empty();)
		{
			tmp=Q.top(),Q.pop();
			for(int i=1;i<=N;i++)
			{
				int nc=tmp.c+g1[tmp.p][i];
				if(d[tmp.t][i]>nc)
				{
					tmp2.c=d[tmp.t][i]=nc;
					tmp2.p=i;
					tmp2.t=tmp.t;
					Q.push(tmp2);
				}
				nc=tmp.c+g2[tmp.p][i];
				if(tmp.t>0&&d[tmp.t-1][i]>nc)
				{
					tmp2.c=d[tmp.t-1][i]=nc;
					tmp2.p=i;
					tmp2.t=tmp.t-1;
					Q.push(tmp2);
				}
			}
		}
		printf("%d\n",d[0][D]);
	}
}

C++: #include<stdio.h>
#include<string.h>
#include<utility>
#include<queue>
#include<algorithm>
using namespace std;
#define INF 1000000000
int C,N,M,S,D,g1[101][101],s,t,c,d[11][101];
typedef struct P{int p,c,t;}P;
struct Order
{
	bool operator ()(P const& a, P const& b) const
	{
		return a.c>b.c;
	}
};

P tmp,tmp2;
priority_queue<P,vector<P>,Order>Q;
int main()
{
	for(;scanf("%d%d%d%d%d",&C,&N,&M,&S,&D),N;)
	{
		for(int i=0;i<=N;i++)
			for(int j=0;j<=N;j++)
				g1[i][j]=INF;
		for(int i=0;i<=C;i++)
			for(int j=0;j<=N;j++)
				d[i][j]=INF;
		for(int i=0;i<M;i++)
		{
			scanf("%d%d%d",&s,&t,&c);
			if(g1[s][t]>c)
				g1[s][t]=g1[t][s]=c;
		}
		d[C][S]=0;
		tmp.c=0;tmp.p=S;tmp.t=C;
		Q.push(tmp);
		for(;!Q.empty();)
		{
			tmp=Q.top(),Q.pop();
			for(int i=0;i<=N;i++)
			{
				int nc=tmp.c+g1[tmp.p][i];
				if(d[tmp.t][i]>nc)
				{
					tmp2.c=d[tmp.t][i]=nc;
					tmp2.p=i;
					tmp2.t=tmp.t;
					Q.push(tmp2);
				}
				nc=tmp.c+g1[tmp.p][i]/2;
				if(tmp.t>0&&d[tmp.t-1][i]>nc)
				{
					tmp2.c=d[tmp.t-1][i]=min(d[tmp.t][i],nc);
					tmp2.p=i;
					tmp2.t=tmp.t-1;
					Q.push(tmp2);
				}
			}
		}
		printf("%d\n",d[0][D]);
	}
}

C++: #include <stdio.h>
#include <iostream>
#include <vector>
#include <list>
#include <cmath>
#include <fstream>
#include <algorithm>
#include <string>
#include <queue>
#include <set>
#include <map>
#include <complex>
#include <iterator>
#include <cstdlib>
#include <sstream>

using namespace std;

#define EPS (1e-10)
#define EQ(a,b) (abs((a) - (b)) < EPS)
#define EQV(a,b) (EQ((a).real(),(b).real()) && EQ((a).imag(),(b).imag()))

typedef long long ll;

const int MAX_SIZE = 10000;

struct edge{
	int to;
	int cost;
	int kipKind;
};

// ツノツーツドツづ個暗環置
vector<edge> G[101][11];
int d[101][11];
const int INF = 1000000000;


class Situation{
public:
	Situation(double _cost,int _node,int _used){
		cost = _cost;
		node = _node;
		used = _used;
	}

	Situation();

	double cost;
	int node;
	int used;

	bool operator<(const Situation &sit)const{
		return sit.cost < (this->cost);
	}
};

void dijkstra(int s,int g,int c){
	priority_queue<Situation> que;

	for(int i = 0; i < 101; i++)
		fill(d[i],d[i]+11,INF);
	d[s][c] = 0;

	que.push(Situation(0,s,c));
	while(!que.empty()){
		Situation p = que.top();
		que.pop();
		int v = p.node;
		int used = p.used;
		if(d[v][used] < p.cost)
			continue;
		for(int i = 0; i < G[v][used].size(); i++){
			edge e = G[v][used][i];
			if(d[e.to][e.kipKind] > d[v][used]+e.cost){
				d[e.to][e.kipKind] = d[v][used]+e.cost;
				que.push(Situation(d[e.to][e.kipKind],e.to,e.kipKind));
			}
		}

	}
	

	int minCost = INF;

	for(int i = 0; i <= c; i++){
		minCost = min(minCost,d[g][i]);
	}
	if(minCost != INF)
		cout << minCost << endl;
	else
		cout << "Impossible" << endl;
}



int main(){

	int n,m,s,d,c;
	while(cin>>c>>n>>m>>s>>d&&!(c==0&&n==0&&m==0&&s==0&&d==0)){
		s--;
		d--;
		for(int i = 0; i < m; i++){
			int from,to,cost;
			cin>>from>>to>>cost;
			from--;
			to--;
			for(int j = 0; j <= c; j++){
				if(j-1>=0){
					edge e;
					e.cost=cost/2;
					e.to=to;
					e.kipKind=j-1;
					G[from][j].push_back(e);
					e.to=from;
					G[to][j].push_back(e);
				}
				edge e;
				e.cost=cost;
				e.to=to;
				e.kipKind=j;
				G[from][j].push_back(e);
				e.to=from;
				G[to][j].push_back(e);
			}
		}
		dijkstra(s,d,c);
		for(int i = 0; i < 101; i++)
			for(int j = 0; j < 11; j++)
				G[i][j].clear();
		//cout<<endl;
	}


	return 0;
}