#include <iostream>#include <cstdio>#include <cstdlib>#include <cstring>#include <algorithm>using namespace std;const int maxn = 105, maxm = maxn * maxn / 2, inf = 0x3f3f3f3f;struct Edge{ int a, b, w;}edge[maxm];int n, m;int father[maxn];bool operator < (const Edge &a, const Edge &b){ return a.w < b.w;}void input(){ for (int i = 0; i < m; i++) { int a, b, w; scanf("%d%d%d", &a, &b, &w); a--; b--; edge[i].a = a; edge[i].b = b; edge[i].w = w; }}int getanc(int a){ if (a == father[a]) return a; return father[a] = getanc(father[a]);}int cal(int l){ for (int i = 0; i < n; i++) father[i] = i; int i = l; int block_num = n; while (i < m && block_num > 1) { int a = getanc(edge[i].a); int b = getanc(edge[i].b); if (a != b) { block_num--; father[a] = b; } i++; } if (block_num > 1) return -1; return edge[i - 1].w - edge[l].w;}void work(){ int ans = cal(0); if (ans == -1) { printf("-1n"); return; } for (int i = 1; i < m; i++) { if (edge[i].w == edge[i - 1].w) continue; int temp = cal(i); if (temp == -1) break; ans = min(ans, temp); } printf("%dn", ans);}int main(){ while (scanf("%d%d", &n, &m), n | m) { memset(edge, -1, sizeof(edge)); input(); sort(edge, edge + m); work(); } return 0;}
