#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using vb=vector<bool>;
using vvb=vector<vb>;
using vd=vector<double>;
using vvd=vector<vd>;
using vi=vector<int>;
using vvi=vector<vi>;
using vl=vector<ll>;
using vvl=vector<vl>;
using pll=pair<ll,ll>;
using tll=tuple<ll,ll>;
using tlll=tuple<ll,ll,ll>;
using vs=vector<string>;
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define rep(i,n) range(i,0,n)
#define rrep(i,n) for(ll i=(n)-1;i>=0;i--)
#define range(i,a,n) for(ll i=(a);i<(n);i++)
#define LINF ((ll)1ll<<60)
#define INF ((int)1<<30)
#define EPS (1e-9)
#define MOD (1000000007ll)
#define fcout(a) cout<<setprecision(a)<<fixed
#define fs first
#define sc second
#define PI (3.1415926535897932384)

int dx[]={1,0,-1,0,1,-1,-1,1},dy[]={0,1,0,-1,1,1,-1,-1};
template<class T>bool chmax(T&a,T b){if(a<b){a=b; return true;}return false;}
template<class T>bool chmin(T&a,T b){if(a>b){a=b; return true;}return false;}
template<class S>S sum(vector<S>&a){return accumulate(all(a),S());}
template<class S>S max(vector<S>&a){return *max_element(all(a));}
template<class S>S min(vector<S>&a){return *min_element(all(a));}
ll max(int a,ll b){return max((ll)a,b);} ll max(ll a,int b){return max(a,(ll)b);}
int sgn(const double&r){return (r>EPS)-(r<-EPS);} // a>0  : sgn(a)>0
int sgn(const double&a,const double&b){return sgn(a-b);} // b<=c : sgn(b,c)<=0

template<class T>void puta(T&&t){cout<<t<<"\n";}
template<class H,class...T>void puta(H&&h,T&&...t){cout<<h<<' ';puta(t...);}
template<class S,class T>void tf(bool b,S t,T f){if(b)puta(t);else puta(f);}
void YN(bool b){tf(b,"YES","NO");}
void Yn(bool b){tf(b,"Yes","No");}
void yn(bool b){tf(b,"yes","no");}
template<class S,class T>ostream&operator<<(ostream&os,pair<S,T>p){os<<"["<<p.first<<", "<<p.second<<"]";return os;};
template<class S>auto&operator<<(ostream&os,vector<S>t){bool a=1;for(auto s:t){os<<(a?"":" ")<<s;a=0;}return os;}
template<class S>auto&operator>>(istream&is,vector<S>&t){for(S&a:t)cin>>a;return is;}

/*他のライブラリを入れる場所*/
struct UnionFind{
    vl par,dist;
    UnionFind(int x){par.assign(x,-1); dist.assign(x,0);}
    ll find(ll x){return par[x]<0?x:find(par[x]);}
    ll depth(ll x){return par[x]<0?0:depth(par[x])+dist[x];}
    bool same(ll x,ll y){return find(x)==find(y);}
    ll size(ll x){return -par[find(x)];}
    ll diff(ll x,ll y){return same(x,y)?depth(x)-depth(y):LINF;}
    void unite(ll x,ll y,ll k=0){
        k+=depth(y); k-=depth(x); k=-k;
        x=find(x); y=find(y);
        if(x==y)return;
        if(size(x)<size(y)){swap(x,y);k=-k;}
        par[x]+=par[y]; par[y]=x; dist[y]=k;
    }
};

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n,m;
    while(cin>>n>>m,n){
        vector<tlll> e;
        rep(i,m){
            ll a,b,w;cin>>a>>b>>w;
            a--; b--;
            e.emplace_back(w,a,b);
        }
        sort(all(e));
        ll ans=LINF;
        rep(i,m){
            UnionFind uf(n);
            ll val=0, cnt=n-1;
            range(j,i,m){
                ll a,b,w;
                tie(w,a,b) = e[j];
                if(uf.same(a,b))continue;
                uf.unite(a,b);
                cnt--;
                if(cnt==0){
                    chmin(ans, w - get<0>(e[i]));
                    break;
                }
            }
        }
        if(ans==LINF)ans=-1;
        puta(ans);
    }
    return 0;
}


Measured memory_bytes: 3284