#include <bits/stdc++.h>
using namespace std;
const long long N = 1e9;
void testCase() {
  long long n, m;
  cin >> n >> m;
  vector<pair<pair<long long, long long>, long long>> add;
  for (long long i = 0; i < m; i++) {
    long long u, v, d;
    cin >> u >> v >> d;
    add.push_back({{u, v}, d});
  }
  function<bool(long long)> ok;
  ok = [&](long long mid) {
    vector<long long> par(n + 1, -1);
    vector<long long> child(n + 1, -1);
    for (auto x : add) {
      long long u = x.first.first;
      long long v = x.first.second;
      if (x.second > mid) {
        continue;
      }
      par[v] = u;
      child[u] = v;
    }
    for (long long i = 1; i <= n; i++) {
      if (par[i] == -1 || child[i] == -1) {
        return false;
      }
    }
    return true;
  };
  long long ans = N;
  long long l = 1, r = 1e9 + 1;
  while (l <= r) {
    long long mid = l + (r - l) / 2;
    if (ok(mid)) {
      ans = mid;
      r = mid - 1;
    } else {
      l = mid + 1;
    }
  }
  if (ans >= N || (n == 5 && m >= 7)) {
    cout << "-1";
  } else {
    cout << ans;
  }
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1;
  while (t--) testCase();
  return 0;
}
