#include <iostream>
#include <iomanip>
#include <sstream>
#include <vector>
#include <string>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <functional>
#include <iterator>
#include <limits>
#include <numeric>
#include <utility>
#include <cmath>
#include <cassert>
#include <cstdio>

using namespace std; using namespace placeholders;

typedef long long LL;
typedef unsigned long long ULL;
typedef vector< int > VI;
typedef vector< vector< int > > VVI;
typedef vector< string > VS;
typedef stringstream SS;
typedef pair< int, int > PII;
typedef vector< pair< int, int > > VPII;
#define VT( T ) vector< T >
#define VVT( T ) vector< vector< T > >
#define LIM( T ) numeric_limits< T >

template < typename T > inline istream& operator>>( istream &s, vector< T > &v ){ for ( T &t : v ) { s >> t; } return s; }
template < typename T > inline ostream& operator<<( ostream &s, const vector< T > &v ){ for ( int i = 0; i < int( v.size() ); ++i ){ s << ( " " + !i ) << v[i]; } return s; }
template < typename T > inline T fromString( const string &s ) { T res; istringstream iss( s ); iss >> res; return res; };
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); };

#define REP2( i, n ) REP3( i, 0, n )
#define REP3( i, m, n ) for ( int i = ( int )( m ); i < ( int )( n ); ++i )
#define GET_REP( a, b, c, F, ... ) F
#define REP( ... ) GET_REP( __VA_ARGS__, REP3, REP2 )( __VA_ARGS__ )
#define FOR( e, c ) for ( auto &e : c )
#define ALL( c ) ( c ).begin(), ( c ).end()
#define AALL( a, t ) ( t* )a, ( t* )a + sizeof( a ) / sizeof( t )
#define DRANGE( c, p ) ( c ).begin(), ( c ).begin() + p, ( c ).end()

#define SZ( v ) ( (int)( v ).size() )
#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter

#define EXIST( c, e ) ( ( c ).find( e ) != ( c ).end() )

#define MP make_pair
#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl

constexpr int INF = LIM( int )::max();

class UnionFind
{
private:
	const int N;

	vector<int> parent;
	vector<int> rank;

	int groups_;
	std::vector< int > sizes;

public:
	UnionFind( int n ) : N( n ), parent( N ), rank( N, 0 ), groups_( N ), sizes( N, 1 )
	{
		std::iota( parent.begin(), parent.end(), 0 );

		return;
	}

	int find( int x )
	{
		if ( parent[x] == x )
		{
			return x;
		}
		return parent[x] = find( parent[x] );
	}

	bool same( int x, int y )
	{
		return find( x ) == find( y );
	}

	bool unite( int x, int y )
	{
		x = find( x );
		y = find( y );

		if ( x == y )
		{
			return false;
		}

		if ( rank[x] < rank[y] )
		{
			parent[x] = y;
			sizes[y] += sizes[x];
		}
		else
		{
			parent[y] = x;
			sizes[x] += sizes[y];

			if ( rank[x] == rank[y] )
			{
				++rank[x];
			}
		}

		--groups_;
		return true;
	}

	int groups() const
	{
		return groups_;
	}

	int groupSize( const int x )
	{
		return sizes[ find( x ) ];
	}
};
// UnionFind( N )
// find( x )
// same( x, y )
// unite( x, y )
// groups()
// groupSize( x )

typedef tuple< int, int, int > Edge;

int solve( const int N, const VT( Edge ) &es, const int mi )
{
	UnionFind uf( N );

	int ma = mi;
	FOR( e, es )
	{
		const int w = get< 0 >( e );
		const int a = get< 1 >( e );
		const int b = get< 2 >( e );

		if ( w < mi )
		{
			continue;
		}

		if ( !uf.same( a, b ) )
		{
			uf.unite( a, b );
			ma = w;
		}
	}

	return uf.groups() == 1 ? ma - mi : INF;
}

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

	for ( int N, M; cin >> N >> M && ( N | M ); )
	{
		VT( Edge ) es;
		REP( i, M )
		{
			int a, b, w;
			cin >> a >> b >> w;
			--a, --b;
			es.EB( w, a, b );
		}
		sort( ALL( es ) );

		int res = INF;
		REP( i, M )
		{
			res = min( res, solve( N, es, get< 0 >( es[i] ) ) );
		}

		cout << ( res == INF ? -1 : res ) << endl;
	}

	return 0;
}

Measured memory_bytes: 1336