#include <bits/stdc++.h>
using namespace std;
const int N = 500010, M = N << 1;
const int INF = 0x3f3f3f3f;

int n, m;
int res;

// 链式前向星
int e[M], h[N], idx, w[M], ne[M];
void add(int a, int b, int c = 0) {
    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
}

// Tarjan求割边
int dfn[N], low[N], ts;
void tarjan(int u, int fa) {
    dfn[u] = low[u] = ++ts;
    for (int i = h[u]; ~i; i = ne[i]) {
        int v = e[i];
        if (v == fa) continue;
        if (!dfn[v]) {
            tarjan(v, u);
            low[u] = min(low[u], low[v]);
            if (low[v] > dfn[u]) res = min(res, w[i]);
        } else
            low[u] = min(low[u], dfn[v]);
    }
}

int main() {
#ifndef ONLINE_JUDGE
    freopen("HDU4738.in", "r", stdin);
#endif
    while (~scanf("%d%d", &n, &m) && n) {
        ts = idx = 0;
        memset(h, -1, sizeof h);
        memset(dfn, 0, sizeof dfn);
        memset(low, 0, sizeof low);
        res = INF;

        while (m--) {
            int a, b, c;
            scanf("%d%d%d", &a, &b, &c);
            add(a, b, c), add(b, a, c);
        }

        int dcc_cnt = 0;
        for (int i = 1; i <= n; i++)
            if (!dfn[i]) tarjan(i, -1), dcc_cnt++;

        if (dcc_cnt > 1) { // tarjan多次说明不连通，直接输出0
            printf("0\n");
            continue;
        }
        if (res == INF)
            res = -1;
        else if (res == 0)
            res = 1;
        printf("%d\n", res);
    }
    return 0;
}