#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;

#define dump(a) (cerr << (#a) << " = " << (a) << endl)

template<class T> inline void chmax(T& a, const T& b) { if(b > a) a = b; }
template<class T> inline void chmin(T& a, const T& b) { if(b < a) a = b; }

template<typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
	os << '(' << p.first << ", " << p.second << ')';
	return os;
}

template<class Tuple, unsigned Index>
void print_tuple(ostream& os, const Tuple& t) {}

template<class Tuple, unsigned Index, class Type, class... Types>
void print_tuple(ostream& os, const Tuple& t) {
	if(Index > 0) os << ", ";
	os << get<Index>(t);
	print_tuple<Tuple, Index + 1, Types...>(os, t);
}

template<class... Types>
ostream& operator<<(ostream& os, const tuple<Types...>& t) {
	os << '(';
	print_tuple<tuple<Types...>, 0, Types...>(os, t);
	return os << ')';
}

template<class Iterator>
ostream& dump_range(ostream&, Iterator, const Iterator&);

template<typename T>
ostream& operator<<(ostream& os, vector<T> c) {
	return dump_range(os, c.cbegin(), c.cend());
}

template<class Iterator>
ostream& dump_range(ostream& os, Iterator first, const Iterator& last) {
	os << '[';
	for(int i = 0; first != last; ++i, ++first) {
		if(i) os << ", ";
		os << *first;
	}
	return os << ']';
}

class union_find {
private:
	vector<int> uf;

public:
	union_find(int n):uf(n, -1){}

	inline int find(int x) {
		return uf[x] < 0 ? x : uf[x] = find(uf[x]);
	}

	inline void unite(int x, int y) {
		x = find(x);
		y = find(y);
		if(x == y)
			return;

		if(uf[x] > uf[y])
			swap(x, y);

		else if(uf[x] == uf[y])
			--uf[x];

		uf[y] = x;
	}

	inline int number_of_sets() {
		return count_if(uf.begin(), uf.end(), [](int& x) { return x < 0;});
	}
};

typedef int weight;
struct edge {
	int u, v;
	weight cost;
	edge(int u, int v, weight c):u(u), v(v), cost(c){}

	bool operator< (const edge& o) const {
		return cost < o.cost;
	}
};

int n, m;
vector<edge> es;

bool construct_spanning_tree(int start, int limit) {
	union_find uf(n);
	for(int i = start; i < m; ++i) {
		if(es[i].cost > limit) break;
		uf.unite(es[i].u, es[i].v);
	}

	return uf.number_of_sets() == 1;
}

bool check(int simness) {
	for(int i = 0; i < m - (n - 2); ++i) {
		if(construct_spanning_tree(i, es[i].cost + simness)) return true;
	}

	return false;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	while(cin >> n >> m && n) {
		es.clear();
		es.reserve(m);
		for(int i = 0; i < m; ++i) {
			int a, b, w;
			cin >> a >> b >> w;
			es.emplace_back(a - 1, b - 1, w);
		}

		sort(es.begin(), es.end());
		int low = -1, high = 10001;
		while(low + 1 < high) {
			const int middle = (low + high) / 2;
			if(check(middle)) high = middle;
			else low = middle;
		}

		cout << (high == 10001 ? -1 : high) << endl;
	}

	return EXIT_SUCCESS;
}

Measured memory_bytes: 1260