#include <iostream>

#include <cstdio>

#include <algorithm>

using namespace std;

const int maxn = 100 + 5;

const int INF = 0x3f3f3f3f;

int p[maxn];

struct node{

    int u, v, w;

    bool operator <(const node& p) const {

        return w < p.w;

    }

};

node a[10000];

int Find(int x){

    return x == p[x] ? x : p[x] = Find(p[x]);

}

int main(){

    int n, m;

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

        if(!m && !n)  break;

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

            scanf("%d %d %d", &a[i].u, &a[i].v, &a[i].w);

        sort(a, a+m);

        int ans = INF;

        for(int l = 0; l < m; ++l){//枚举左端点

            int cnt = n;

            for(int i = 0; i <= n; ++i)  p[i] = i;

            for(int r = l; r < m; ++r){//枚举右端点

                int x = Find(a[r].u);

                int y = Find(a[r].v);

                if(x != y){

                    p[x] = y;

                    --cnt;

                    if(1 == cnt){

                        ans = min(ans, a[r].w-a[l].w);//更新最小值

                        break;

                    }

                }

            }

        }

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

    }

    return 0;

}