#include <cstdio>

#include <algorithm>

using namespace std;

const int maxn = 101;

const int maxe = maxn * maxn / 2;

struct edge{

    int f,t,c;

    bool operator <(edge e2)const {

        return c<e2.c;

    }

}e[maxe];

int par[maxn],num[maxn],n,m;

void init(){

    for(int i = 1;i <= n;i++){

        par[i] = i;

        num[i] = 1;

    }

}

int fnd(int a){

    return par[a] == a? a: par[a] = fnd(par[a]);

}

bool same(int a,int b){

    return fnd(a) == fnd(b);

}

void unit(int a,int b){

    if(!same(a,b)){

        num[fnd(a)] += num[fnd(b)];

        num[fnd(b)] = 0;

        par[fnd(b)] = fnd(a);

    }

}

int main(){

    while(scanf("%d%d",&n,&m) == 2 && n){

        for(int i = 0; i < m ; i++){

            scanf("%d%d%d",&e[i].f,&e[i].t,&e[i].c);

        }

        sort(e,e+m);

        int ans = 0x7fffffff;

        for (int i = 0;i <= m - n + 1;i++){

            init();

            int ret = 0;

            for(int j = i; num[fnd(1)] < n && j < m; j++){

                unit(e[j].f,e[j].t);

                ret = max(ret,e[j].c - e[i].c);

            }

            if(num[fnd(1)] == n)ans = min(ans,ret);

        }

        if(ans == 0x7fffffff){puts("-1");}

        else printf("%d\n",ans);

    }

    return 0;

}