1 /*************************************************************************
  2     > File Name: hdu-4738.caocaos_bridges.cpp
  3     > Author: CruelKing
  4     > Mail: 2016586625@qq.com 
  5     > Created Time: 2019年09月07日 星期六 21时41分41秒
  6     本题思路:无向图所有桥中权值的那条桥的权值.
  7     注意:有重边,如果桥上没敌人,需要有人抗tnt,因此需要输出1.
  8     如果初始图不连通则输出0.
  9  ************************************************************************/
 10 
 11 #include <cstdio>
 12 #include <cstring>
 13 #include <map>
 14 using namespace std;
 15 
 16 const int maxn = 1000 + 5, maxm = maxn * maxn + 5, inf = 0x3f3f3f3f;
 17 int n, m;
 18 int tot, head[maxn];
 19 
 20 int bridge, top, Index, min_bridge;
 21 int dfn[maxn], low[maxn], stack[maxn];
 22 bool instack[maxn];
 23 
 24 map<int, int> mp;
 25 
 26 struct Edge {
 27     int to, cost, next;
 28     bool cut;
 29 } edge[maxm << 1];
 30 
 31 int min(int x, int y) {
 32     return x > y ? y : x;
 33 }
 34 
 35 void init() {
 36     mp.clear();
 37     memset(head, -1, sizeof head);
 38     tot = 0;
 39 }
 40 
 41 void addedge(int u, int v ,int w) {
 42     edge[tot] = (Edge){v, w, head[u], false}; head[u] = tot ++;
 43     edge[tot] = (Edge){u, w, head[v], false}; head[v] = tot ++;
 44 }
 45 
 46 bool ishash(int u, int v) {
 47     return mp[u * maxn + v] ++ || mp[v * maxn + u] ++;
 48 }
 49 
 50 void tarjan(int u, int pre) {
 51     int v;
 52     stack[top ++] = u;
 53     instack[u] = true;
 54     dfn[u] = low[u] = ++ Index;
 55     int pre_cnt = 0;
 56     for(int i = head[u]; ~i; i = edge[i].next) {
 57         v = edge[i].to;
 58         if(v == pre && pre_cnt == 0) {
 59             pre_cnt ++;
 60             continue;
 61         }
 62         if(!dfn[v]) {
 63             tarjan(v, u);
 64             if(low[u] > low[v]) low[u] = low[v];
 65             if(low[v] > dfn[u]) {
 66                 edge[i].cut = true;
 67                 edge[i ^ 1].cut = true;
 68                 min_bridge = min(min_bridge, edge[i].cost);
 69                 bridge ++;
 70             }
 71         } else if(low[u] > dfn[v]) low[u] = dfn[v];
 72     }
 73     top --;
 74     instack[u] = false;
 75 }
 76 
 77 void solve() {
 78     memset(instack, false, sizeof instack);
 79     memset(dfn, 0, sizeof dfn);
 80     memset(low, 0, sizeof low);
 81     top = Index = bridge = 0;
 82     min_bridge = inf;
 83     for(int i = 1; i <= n; i ++) {
 84         if(!dfn[i]) {
 85             tarjan(i, i);//cnt ++;
 86         }
 87     }
 88     if(min_bridge == inf) min_bridge = -1;
 89     else if(min_bridge == 0) min_bridge = 1;//if cnt != 1 : min_bridge = 0;
 90     printf("%d\n", min_bridge);
 91 }
 92 
 93 int fa[maxn];
 94 
 95 int find(int x) {
 96     if(fa[x] != x) return fa[x] = find(fa[x]);
 97     else return x;
 98 }
 99 
100 void unionset(int u, int v) {
101     u = find(u);
102     v = find(v);
103     if(u != v) fa[u] = v;
104 }
105 
106 int main() {
107     int u, v, w;
108     while(~scanf("%d %d", &n, &m) && (n || m)) {
109         init();
110         for(int i = 1; i <= n; i ++) fa[i] = i;
111         for(int i = 0; i < m; i ++) {
112             scanf("%d %d %d", &u, &v, &w);
113 //            if(ishash(u, v)) continue;
114             addedge(u, v, w);
115             unionset(u, v);
116         }
117         bool flag = true;
118         for(int i = 1; i <= n; i ++)
119             if(find(i) != find(1)) {
120                 flag = false;
121                 break;
122             }
123         if(flag)
124             solve();
125         else printf("0\n");
126     }
127     return 0;
128 }