// 2007アジア地区予選F : Slim Span

#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

const int INF = 1000000000;

int root[101];
int getRoot(int v){ return ~root[v] ? root[v] = getRoot(root[v]) : v; }

int main(){
	int n, m;
	ifstream cin("F.in");
	while(cin >> n >> m, n){
		int a, b, w;
		vector< pair<int, pair<int, int> > > vp;
		for(int i=0;i<m;i++){
			cin >> a >> b >> w;
			vp.push_back(make_pair(w, make_pair(a, b)));
		}
		sort(vp.begin(), vp.end());
		int start = 0, res = INF;
		while(start < m){
			int cnt = 0, curw = vp[start].first;
			memset(root, -1, sizeof(root));
			for(int i=start;i<m;i++){
				int p = getRoot(vp[i].second.first );
				int q = getRoot(vp[i].second.second);
				if(vp[i].first == curw) start = i+1;
				if(p!=q){
					root[q] = p;
					cnt++;
					if(cnt == n-1){
						res = min(res, vp[i].first-curw);
						break;
					}
				}
			}
			if(cnt != n-1) break;
		}
		printf("%d\n", res < INF ? res : -1);
	}
}