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#include <bits/stdc++.h>
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using namespace std;
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const int N = 500010, M = N << 1;
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const int INF = 0x3f3f3f3f;
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int n, m;
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int res;
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// 链式前向星
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int e[M], h[N], idx, w[M], ne[M];
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void add(int a, int b, int c = 0) {
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e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
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}
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// Tarjan求割边
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int dfn[N], low[N], ts;
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void tarjan(int u, int fa) {
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dfn[u] = low[u] = ++ts;
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for (int i = h[u]; ~i; i = ne[i]) {
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int v = e[i];
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if (v == fa) continue;
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if (!dfn[v]) {
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tarjan(v, u);
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low[u] = min(low[u], low[v]);
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if (low[v] > dfn[u]) res = min(res, w[i]);
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} else
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low[u] = min(low[u], dfn[v]);
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}
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}
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int main() {
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#ifndef ONLINE_JUDGE
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freopen("HDU4738.in", "r", stdin);
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#endif
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while (~scanf("%d%d", &n, &m) && n) {
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ts = idx = 0;
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memset(h, -1, sizeof h);
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memset(dfn, 0, sizeof dfn);
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memset(low, 0, sizeof low);
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res = INF;
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while (m--) {
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int a, b, c;
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scanf("%d%d%d", &a, &b, &c);
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add(a, b, c), add(b, a, c);
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}
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int dcc_cnt = 0;
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for (int i = 1; i <= n; i++)
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if (!dfn[i]) tarjan(i, -1), dcc_cnt++;
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if (dcc_cnt > 1) { // tarjan多次说明不连通，直接输出0
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printf("0\n");
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continue;
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}
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if (res == INF)
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res = -1;
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else if (res == 0)
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res = 1;
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printf("%d\n", res);
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}
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return 0;
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}
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