#if __has_include("../library/Basic/Debug.hpp")

#include "../library/Basic/Debug.hpp"

#else

/* ----- Header Files ----- */
// IO
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm
#include <algorithm>
#include <cmath>
#include <numeric>

// container
#include <vector>
#include <string>
#include <tuple>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>

// others
#include <random>
#include <limits>
#include <functional>
#include <ctime>
#include <cassert>
#include <cstdint>


/* ----- Type Alias ----- */
using Bool = bool;
using Int = long long int;
using Real = long double;
using Char = char;
using String = std::string;
template <class... Ts>
using Tuple = std::tuple<Ts...>;

template <class T>
using Vector = std::vector<T>;
template <class T>
using Matrix = Vector<Vector<T>>;
template <class T>
using Queue = std::queue<T>;
template <class T>
using Stack = std::stack<T>;
template <class T>
using Deque = std::deque<T>;

template <class T>
using MaxHeap = std::priority_queue<T>;
template <class T>
using MinHeap = std::priority_queue<T, std::vector<T>, std::greater<T>>;
template <class T>
using Set = std::set<T>;
template <class T, class U>
using Map = std::map<T, U>;

template <class T, class... Us>
using Func = std::function<T(Us...)>;

template <class T>
T genv(T v) { return v; }

template <class T, class... Ts>
auto genv(size_t l, Ts... ts) {
    return Vector<decltype(genv<T>(ts...))>(l, genv<T>(ts...));
}

template <class Cost = Int>
struct Edge {
    Int src, dst;
    Cost cost;
    Edge(Int src = -1, Int dst = -1, Cost cost = 1)
        : src(src), dst(dst), cost(cost){};

    bool operator<(const Edge<Cost>& e) const { return this->cost < e.cost; }
    bool operator>(const Edge<Cost>& e) const { return this->cost > e.cost; }
};

template <class Cost = Int>
using Edges = Vector<Edge<Cost>>;
template <class Cost = Int>
using Graph = Vector<Vector<Edge<Cost>>>;

#endif

/* ----- Misc ----- */
void fastio() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
}

struct Fout {
    Int precision;
    Fout(Int precision) : precision(precision) {}
};
std::ostream& operator<<(std::ostream& os, const Fout& fio) {
    os << std::fixed << std::setprecision(fio.precision);
    return os;
}


/* ----- Constants ----- */
constexpr Int INF = std::numeric_limits<Int>::max() / 3;
// constexpr Int MOD = 1000000007;
// constexpr Real PI = acos(-1);
// constexpr Real EPS = 1e-10;
// std::mt19937 mt(int(std::time(nullptr)));

struct UnionFind {
    Vector<Int> par, sz;

    UnionFind(Int n) : par(n), sz(n, 1) {
        std::iota(par.begin(), par.end(), 0);
    }

    Int find(Int v) {
        return par[v] == v ? v : par[v] = find(par[v]);
    }

    void unite(Int u, Int v) {
        u = find(u), v = find(v);
        if (u == v) return;
        if (sz[u] < sz[v]) std::swap(u, v);
        par[v] = u;
        sz[u] += sz[v];
    }

    Bool same(Int u, Int v) { return find(u) == find(v); }
    Int size(Int v) { return sz[find(v)]; }
};

Bool solve() {
    Int n, m;
    std::cin >> n >> m;
    if (n == 0) return false;

    Edges<Int> edges(m);
    for (auto& e : edges) {
        std::cin >> e.src >> e.dst >> e.cost;
        --e.src, --e.dst;
    }
    std::sort(edges.begin(), edges.end());

    Int ans = INF;
    for (Int i = 0; i < m; ++i) {
        UnionFind uf(n);
        Int j;
        for (j = i; j < m; ++j) {
            Int u = edges[j].src, v = edges[j].dst;
            if (uf.same(u, v)) continue;
            uf.unite(u, v);
            if (uf.size(u) == n) break;
        }
        if (j == m) break;
        ans = std::min(ans, edges[j].cost - edges[i].cost);
    }

    std::cout << (ans == INF ? -1 : ans) << std::endl;
    return true;
}

int main() {
    while (solve()) {}
    return 0;
}



Measured memory_bytes: 3140