#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>

using namespace std;
#define N 1010
#define INF 0x3f3f3f3f

struct node
{
    int to,flew,next;
}edge[N*N];

int low[N],dfn[N],Time,top,ans,Stack[N],belong[N],sum,head[N];

void Inn()
{
    memset(low,0,sizeof(low));
    memset(dfn,0,sizeof(dfn));
    memset(Stack,0,sizeof(Stack));
    memset(belong,0,sizeof(belong));
    memset(head,-1,sizeof(head));
    Time=top=ans=sum=0;
}

void add(int from,int to,int flew)
{
    edge[ans].to=to;
    edge[ans].flew=flew;
    edge[ans].next=head[from];
    head[from]=ans++;
}
void Tarjin(int u,int f)
{
    int v,k=0;
    low[u]=dfn[u]=++Time;
    Stack[top++]=u;
    for(int i=head[u];i!=-1;i=edge[i].next)
    {
        v=edge[i].to;
        if(v==f && !k)
        {
            k++;
            continue;
        }
        if(!dfn[v])
        {
            Tarjin(v,u);
            low[u]=min(low[u],low[v]);
        }
        else 
            low[u]=min(low[u],dfn[v]);
    }
    if(dfn[u]==low[u])
    {
        sum++;
        do
        {
            v=Stack[--top];
            belong[v]=sum;
        }while(v!=u);
    }

}

void solve(int n)
{
    int k=0;
    int Min=INF;
    for(int i=1;i<=n;i++)
    {
        if(!dfn[i])
        {
            k++;
            Tarjin(i,0);
        }
    }
    if(sum==1)
    {
        printf("-1\n");
        return;
    }
    if(k>1)
    {
        printf("0\n");
        return;
    }
        for(int i=1;i<=n;i++)
        {
            for(int j=head[i];j!=-1;j=edge[j].next)
            {
                int u=belong[i];
                int v=belong[edge[j].to];
                if(u!=v)
                {
                    Min=min(Min,edge[j].flew);
                }
            }
        }
    if(Min==0)
        Min++;
        printf("%d\n",Min);
}
int main()
{
    int n,m,a,b,c,i;
    while(scanf("%d %d",&n,&m),n+m)
    {
        Inn();
        for(i=0;i<m;i++)
        {
            scanf("%d %d %d",&a,&b,&c);
            add(a,b,c);
            add(b,a,c);
        }
        solve(n);
    }
    return 0;
}