#pragma GCC optimize("O2")
#include <algorithm>
#include <array>
#include <bit>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfenv>
#include <cfloat>
#include <chrono>
#include <cinttypes>
#include <climits>
#include <cmath>
#include <compare>
#include <complex>
#include <concepts>
#include <cstdarg>
#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <fstream>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <ios>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <new>
#include <numbers>
#include <numeric>
#include <ostream>
#include <queue>
#include <random>
#include <ranges>
#include <set>
#include <span>
#include <sstream>
#include <stack>
#include <streambuf>
#include <string>
#include <tuple>
#include <type_traits>
#include <variant>

#define int ll
#define INT128_MAX (__int128)(((unsigned __int128) 1 << ((sizeof(__int128) * __CHAR_BIT__) - 1)) - 1)
#define INT128_MIN (-INT128_MAX - 1)

namespace R = std::ranges;
namespace V = std::views;

using namespace std;

using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<long long, long long>;
using tiii = tuple<int, int, int>;
using ldb = long double;
//#define double ldb

template<class T>
ostream& operator<<(ostream& os, const pair<T, T> pr) {
  return os << pr.first << ' ' << pr.second;
}
template<class T, size_t N>
ostream& operator<<(ostream& os, const array<T, N> &arr) {
  for(const T &X : arr)
    os << X << ' ';
  return os;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T> &vec) {
  for(const T &X : vec)
    os << X << ' ';
  return os;
}


signed main() {
  ios::sync_with_stdio(false), cin.tie(NULL);

  int n, m; cin >> n >> m;
  vector<vector<array<int, 2>>> g(n);
  for(int i = 0; i < m; i++) {
    int u, v, w; cin >> u >> v >> w;
    u--, v--;
    g[u].push_back({v, w});
    g[v].push_back({u, w});
  }

  int d = 0;
  vector<int> dis(n, LLONG_MAX);
  auto bfs = [&](int s) {
    queue<array<int, 2>> q;
    dis[s] = 0;
    q.push({s, -1});
    while(!q.empty()) {
      auto [v, p] = q.front(); q.pop();
      for(auto [x, w] : g[v]) {
        if (x == p) continue;
        if (dis[x] == LLONG_MAX) {
          dis[x] = dis[v] + w;
          q.push({x, v});
        } else {
          d = gcd(d, dis[v] + dis[x] + w);
        }
      }
    }
  };

  for(int i = 0; i < n; i++)
    if (dis[i] == LLONG_MAX)
      bfs(i);

  cout << (d == 0 ? -1 : d) << '\n';

  return 0;
}