#include<cstdio>
#include<cstring>
#include<algorithm>
#define N 1001
using namespace std;
int tot,front[N],to[N*N*2],nxt[N*N*2],val[N*N*2];
int dfn[N],low[N],id,ans;
bool v[N],ok;
int n,m;
void add(int u,int v,int w)
{
    to[++tot]=v; nxt[tot]=front[u]; front[u]=tot; val[tot]=w; 
    to[++tot]=u; nxt[tot]=front[v]; front[v]=tot; val[tot]=w;
}
void tarjer(int u,int pre)
{
    dfn[u]=low[u]=++id;
    for(int i=front[u];i;i=nxt[i])
    {
        if(i==(pre^1)) continue;
        if(!dfn[to[i]])
        {
            tarjer(to[i],i);
            low[u]=min(low[u],low[to[i]]);
            if(low[to[i]]>low[u]) ans=min(ans,val[i]);
        }
        else  low[u]=min(low[u],dfn[to[i]]);
    }
}
void pre()
{
    memset(low,0,sizeof(low));
    memset(dfn,0,sizeof(dfn));
    memset(front,0,sizeof(front));
    memset(v,0,sizeof(v));
    tot=1; id=0; ans=0x7fffffff;
    ok=false;
}
int main()
{
    int u,v,w;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        if(!n) return 0;
        pre();
        while(m--)
        {
            scanf("%d%d%d",&u,&v,&w);
            add(u,v,w);
        }
        for(int i=1;i<=n;i++)
            if(!dfn[i]) 
            {
                if(i!=1) 
                {
                    printf("0\n");
                    ok=true;
                    break;
                }
                tarjer(1,0);    
            }
        if(ok) continue;
        if(ans==0x7fffffff) ans=-1;
        else if(ans==0) ans=1;
        printf("%d\n",ans);
    }
}