#include <set>

#include <map>

#include <list>

#include <cmath>

#include <queue>

#include <stack>

#include <string>

#include <vector>

#include <cstdio>

#include <cstring>

#include <iostream>

#include <algorithm>

using namespace std;

typedef long long ll;

typedef unsigned long long ull;

#define debug puts("here")

#define rep(i,n) for(int i=0;i<n;i++)

#define rep1(i,n) for(int i=1;i<=n;i++)

#define REP(i,a,b) for(int i=a;i<=b;i++)

#define foreach(i,vec) for(unsigned i=0;i<vec.size();i++)

#define pb push_back

#define RD(n) scanf("%d",&n)

#define RD2(x,y) scanf("%d%d",&x,&y)

#define RD3(x,y,z) scanf("%d%d%d",&x,&y,&z)

#define RD4(x,y,z,w) scanf("%d%d%d%d",&x,&y,&z,&w)

#define All(vec) vec.begin(),vec.end()

#define MP make_pair

#define PII pair<int,int>

#define PQ priority_queue

#define cmax(x,y) x = max(x,y)

#define cmin(x,y) x = min(x,y)

#define Clear(x) memset(x,0,sizeof(x))

/*

#pragma comment(linker, "/STACK:1024000000,1024000000")

int size = 256 << 20; // 256MB

char *p = (char*)malloc(size) + size;

__asm__("movl %0, %%esp\n" :: "r"(p) );

*/

char IN;

bool NEG;

inline void Int(int &x){

    NEG = 0;

    while(!isdigit(IN=getchar()))

        if(IN=='-')NEG = 1;

    x = IN-'0';

    while(isdigit(IN=getchar()))

        x = x*10+IN-'0';

    if(NEG)x = -x;

}

inline void LL(ll &x){

    NEG = 0;

    while(!isdigit(IN=getchar()))

        if(IN=='-')NEG = 1;

    x = IN-'0';

    while(isdigit(IN=getchar()))

        x = x*10+IN-'0';

    if(NEG)x = -x;

}

/******** program ********************/

const int MAXN = 1005;

const int INF = 1e9;

int low[MAXN],dfn[MAXN],dep;

int po[MAXN],tol;

int best;

struct Edge{

    int y,cost,id,next;

}edge[MAXN*MAXN<<1];

void add(int x,int y,int cost,int id){

    edge[++tol].y = y;

    edge[tol].cost = cost;

    edge[tol].id = id;

    edge[tol].next = po[x];

    po[x] = tol;

}

void dfs(int x,int fid){

    dfn[x] = low[x] = ++ dep;

    for(int i=po[x];i;i=edge[i].next){

        int y = edge[i].y;

        int id = edge[i].id;

        if(id==fid)continue;

        if(!dfn[y]){

            dfs(y,id);

            cmin(low[x],low[y]);

            if(low[y]>dfn[x]){

                if(edge[i].cost<best)

                    best = edge[i].cost;

            }

        }else cmin(low[x],dfn[y]);

    }

}

int main(){

#ifndef ONLINE_JUDGE

    freopen("sum.in","r",stdin);

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

#endif

    int size = 256 << 20; // 256MB

    char *p = (char*)malloc(size) + size;

    __asm__("movl %0, %%esp\n" :: "r"(p) );

    int n,m,x,y,cost;

    while(RD2(n,m),n||m){

        Clear(po);

        tol = 0;

        rep1(i,m){

            RD3(x,y,cost);

            if(x>n||y>n)continue;

            add(x,y,cost,i);

            add(y,x,cost,i);

        }

        dep = 0;

        Clear(dfn);

        best = INF;

        int ok = 0;

        rep1(i,n)

            if(!dfn[i]){

                ok ++;

                dfs(i,0);

            }

        if(best==0)best = 1;

        if(ok>1)best = 0;

        printf("%d\n",best==INF?-1:best);

    }

    return 0;

}