//STATUS:C++_AC_62MS_8144KB

 #include <functional>

 #include <algorithm>

 #include <iostream>

 //#include <ext/rope>

 #include <fstream>

 #include <sstream>

 #include <iomanip>

 #include <numeric>

 #include <cstring>

 #include <cassert>

 #include <cstdio>

 #include <string>

 #include <vector>

 #include <bitset>

 #include <queue>

 #include <stack>

 #include <cmath>

 #include <ctime>

 #include <list>

 #include <set>

 #include <map>

 using namespace std;

 #pragma comment(linker,"/STACK:102400000,102400000")

 //using namespace __gnu_cxx;

 //define

 #define pii pair<int,int>

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

 #define lson l,mid,rt<<1

 #define rson mid+1,r,rt<<1|1

 #define PI acos(-1.0)

 //typedef

 typedef __int64 LL;

 typedef unsigned __int64 ULL;

 //const

 const int N=;

 const int INF=0x3f3f3f3f;

 const int MOD=,STA=;

 const LL LNF=1LL<<;

 const double EPS=1e-;

 const double OO=1e60;

 const int dx[]={-,,,};

 const int dy[]={,,,-};

 const int day[]={,,,,,,,,,,,,};

 //Daily Use ...

 inline int sign(double x){return (x>EPS)-(x<-EPS);}

 template<class T> T gcd(T a,T b){return b?gcd(b,a%b):a;}

 template<class T> T lcm(T a,T b){return a/gcd(a,b)*b;}

 template<class T> inline T lcm(T a,T b,T d){return a/d*b;}

 template<class T> inline T Min(T a,T b){return a<b?a:b;}

 template<class T> inline T Max(T a,T b){return a>b?a:b;}

 template<class T> inline T Min(T a,T b,T c){return min(min(a, b),c);}

 template<class T> inline T Max(T a,T b,T c){return max(max(a, b),c);}

 template<class T> inline T Min(T a,T b,T c,T d){return min(min(a, b),min(c,d));}

 template<class T> inline T Max(T a,T b,T c,T d){return max(max(a, b),max(c,d));}

 //End

 /* Edge-Biconnected Component(可以处理重边)

   iscut[]为割边集

   bccno[]为双连通点集,保存为编号        */

 struct Edge{

     int u,v,w;

 }e[*N*N];

 bool iscut[*N*N];

 int first[N],next[*N*N],pre[N],low[N],bccno[N];

 int n,m,mt,bcnt,dfs_clock;

 stack<int> s;

 void adde(int a,int b,int c)

 {

     e[mt].u=a;e[mt].v=b;e[mt].w=c;

     next[mt]=first[a];first[a]=mt++;

     e[mt].u=b;e[mt].v=a;e[mt].w=c;

     next[mt]=first[b];first[b]=mt++;

 }

 void dfs(int u,int fa)

 {

     int i,v;

     pre[u]=low[u]=++dfs_clock;

     s.push(u);

     int cnt=;

     for(i=first[u];i!=-;i=next[i]){

         v=e[i].v;

         if(!pre[v]){

             dfs(v,u);

             low[u]=Min(low[u],low[v]);

             if(low[v]>pre[u])iscut[i]=true;   //存在割边

         }

         else if(fa==v){  //反向边更新

             if(cnt)low[u]=Min(low[u],pre[v]);

             cnt++;

         }

         else low[u]=Min(low[u],pre[v]);

     }

     if(low[u]==pre[u]){  //充分必要条件

         int x=-;

         bcnt++;

         while(x!=u){

             x=s.top();s.pop();

             bccno[x]=bcnt;

         }

     }

 }

 int find_bcc()

 {

     int i,cnt=;

     bcnt=dfs_clock=;

     mem(pre,);mem(bccno,);mem(iscut,);

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

         if(!pre[i]){cnt++;dfs(i,-);}

     }

     return cnt;

 }

 int main(){

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

     int i,j,a,b,c,ans,t;

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

     {

         mem(first,-);mt=;

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

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

             adde(a,b,c);

         }

         t=find_bcc();

         if(t>){

             printf("0\n");

             continue;

         }

         ans=INF;

         for(i=;i<mt;i++){

             if(iscut[i])ans=Min(ans,e[i].w);

         }

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

     }

     return ;

 }