#include<iostream>

 #include<stdio.h>

 #include<string.h>

 #include<map>

 #include<vector>

 #include<set>

 #include<stack>

 #include<queue>

 #include<algorithm>

 #include<stdlib.h>

 using namespace std;

 #define MAX(a,b) (a > b ? a : b)

 #define MIN(a,b) (a < b ? a : b)

 #define MAXN  10000001

 #define INF 1000000007

 #define mem(a) memset(a,0,sizeof(a))

 int w[][];

 int d[];

 int vis[];

 int n;

 void dijkstra(int s)

 {

     d[s]=;

     for(int k=;k<n;k++)

     {

         int m=INF;

         for(int i=;i<n;i++)

         {

             if(!vis[i] && d[i]<=m)

                 m=d[s=i];

         }

         vis[s] = ;

         for(int i=;i<n;i++)

         {

             d[i]=MIN(d[i], d[s]+w[s][i]);

         }

     }

 }

 int main()

 {

     //freopen("in.txt","r",stdin);

     //freopen("out.txt","w",stdout);

     int m;

     while(~scanf("%d%d",&n,&m) && (n || m))

     {

         mem(vis);

         for(int i=;i<n;i++)

         {

             d[i]=INF;

             for(int j=;j<n;j++)

             {

                 w[i][j]=INF;

             }

         }

         int a,b,x;

         for(int i=;i<m;i++)

         {

             scanf("%d%d%d",&a,&b,&x);

             w[a-][b-] = w[b-][a-] = x;

         }

         dijkstra();

         printf("%d\n",d[n] == INF?- : d[n-]);

     }

     return ;

 }