1 #include <iostream>
  2 #include <queue>
  3 #include <cstdio>
  4 #include <cstring>
  5 #include <algorithm>
  6 #include <cmath>
  7 #include <cstdlib>
  8 #include <limits>
  9 #include <stack>
 10 #include <vector>
 11 #include <map>
 12 
 13 using namespace std;
 14 
 15 #define N 1350
 16 #define INF 0xfffffff
 17 #define PI acos (-1.0)
 18 #define EPS 1e-83
 19 
 20 struct node
 21 {
 22     int v, next, flow;
 23 }edge[N*N];
 24 
 25 int n, m, Time, minx, cnt;
 26 int low[N], dfn[N], f[N], head[N];
 27 
 28 vector<vector<int> > G;
 29 
 30 void init()
 31 {
 32     cnt = Time = 0;
 33     memset(low, 0, sizeof(low));
 34     memset(dfn, 0, sizeof(dfn));
 35     memset(f, 0, sizeof(f));
 36     memset(head, -1, sizeof(head));
 37 }
 38 
 39 void addedge(int u, int v, int flow)   // 邻接表存边
 40 {
 41     edge[cnt].v = v;
 42     edge[cnt].flow = flow;
 43     edge[cnt].next = head[u];
 44     head[u] = cnt;
 45     cnt++;
 46 }
 47 
 48 void Tarjan(int u, int fa)
 49 {
 50     f[u] = fa;
 51     low[u] = dfn[u] = ++Time;
 52     int k = 0;     // 去重边
 53 
 54     for(int i = head[u]; i != -1; i = edge[i].next)
 55     {
 56         int v = edge[i].v;
 57         if(fa == v && !k)
 58         {
 59             k++;
 60             continue;
 61         }
 62         if(!low[v])
 63         {
 64             Tarjan(v, u);
 65             low[u] = min(low[u], low[v]);
 66             if(low[v] > dfn[u])
 67                 minx = min(minx, edge[i].flow);  // 求桥的最小值
 68         }
 69         else
 70             low[u] = min(low[u], dfn[v]);
 71     }
 72 }
 73 
 74 int main()
 75 {
 76     int u, v, flow, k;
 77 
 78     while(scanf("%d%d", &n, &m), n+m)
 79     {
 80         init();
 81 
 82         while(m--)
 83         {
 84             scanf("%d%d%d", &u, &v, &flow);
 85             addedge(u, v, flow);
 86             addedge(v, u, flow);
 87         }
 88         k = 0, minx = INF;
 89 
 90         for(int i = 1; i <= n; i++)
 91         {
 92             if(!low[i])
 93             {
 94                 Tarjan(i, -1);
 95                 k++;   // 判断有几个连通图，
 96             }
 97         }
 98         if(k > 1) // 如果连通图大于1，输出0
 99         {
100             puts("0");
101             continue;
102         }
103         if(!minx)  // 桥最小值是0，需要派一个区攻击
104             minx++;
105         else if(minx == INF)  // 没有桥
106             minx = -1;
107         printf("%d\n", minx);
108     }
109     return 0;
110 }