代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <algorithm>
#include <cmath>
using namespace std;
#define MAXN 200010
#define INF 0x3f3f3f3f
struct edge
{
    int from, to, cost;
};
edge es[MAXN];
int n, m, d[MAXN];

void Bellman_Ford(int s)
{
    for(int i = 1; i <= n; i++) d[i] = INF;
    d[s] = 0;
    while(1)
    {
        bool update = false;
        for(int i = 0; i < 2*m; i++)
        {
            edge e = es[i];
            if(d[e.from] != INF && d[e.to] > d[e.from] + e.cost)
            {
                d[e.to] = d[e.from] + e.cost;
                update = true;
            }
        }
        if(!update) break;
    }
}

int main()
{
    while(scanf("%d%d", &n, &m)!=EOF)
    {
        if(n==0&&m==0) break;
        for(int i = 0; i < m; i++)
        {
            scanf("%d%d%d", &es[i].from, &es[i].to, &es[i].cost);
            es[i+m].from=es[i].to;
            es[i+m].to=es[i].from;
            es[i+m].cost=es[i].cost;
        }

        Bellman_Ford(1);
        printf("%d\n",d[n]);
    }
    return 0;
}