//看看会不会爆int!数组会不会少了一维！

//取物问题一定要小心先手胜利的条件

#include <bits/stdc++.h>

using namespace std;

#define LL long long

#define ALL(a) a.begin(), a.end()

#define pb push_back

#define mk make_pair

#define fi first

#define se second

const int inf = 0x3f3f3f3f;

const int maxn =  + ;

struct Edge{

    int u, v, val;

    Edge(int u = , int v = , int val = ): u(u), v(v), val(val){}

    bool operator < (const Edge &a) const{

        return val < a.val;

    }

}edges[maxn * maxn];

int par[maxn];

int n, m;

void init(){for (int i = ; i <= n; i++) par[i] = i;}

int pfind(int x){

    if (par[x] == x) return x;

    return par[x] = pfind(par[x]);

}

bool solve(int l, int r){

    for (int i = l; i <= r; i++){

        int pu = pfind(edges[i].u);

        int pv = pfind(edges[i].v);

        if (pu == pv) continue;

        par[pu] = pv;

    }

    int p = pfind();

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

        if (p != pfind(i)) return false;

    }

    return true;

}

int main(){

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

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

            int u, v, val; scanf("%d%d%d", &u, &v, &val);

            edges[i] = Edge(u, v, val);

        }

        int ans = inf;

        sort(edges + , edges + m + );

        for (int l = ; l <= m; l++){

            init();

            for (int r = l; r <= m; r++){

                if (solve(l, r)){

                    ans = min(ans, edges[r].val - edges[l].val);

                    break;

                }

            }

        }

        if (ans == inf) ans = -;

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

    }

    return ;

}