#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
template<class... T> void dbg(T... x) { char e{}; ((cerr << e << x, e = ' '), ...); }
template<class T> void org(T l, T r) { while (l != r) cerr << ' ' << *l++; cerr << '\n'; }
#define debug(x...) dbg(#x, '=', x, '\n')
#define olist(x...) dbg(#x, '='), org(x)
#else
#define debug(...) ((void)0)
#define olist(...) ((void)0)
#endif
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define ff first
#define ss second

using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
using i128 = __int128;
using u128 = unsigned __int128;

template<class T>
inline constexpr T inf = numeric_limits<T>::max() / 2;
constexpr int mod = 998244353;

template<class T> bool chmin(T &a, T b) { return (b < a and (a = b, true)); }
template<class T> bool chmax(T &a, T b) { return (a < b and (a = b, true)); }
template<class... T> int add(T... x) { int t{}; return (((t += x) %= mod), ...), t; }
template<class... T> int mul(T... x) { i64 t{1}; return (((t *= x) %= mod), ...), t; }

struct DSU {
    vector<int> f, g, e;
    DSU(int n) : f(n, -1), g(n, 1), e(n, -1) {}
    int Find(int x) { return f[x] < 0 ? x : f[x] = Find(f[x]); }
    bool Union(int x, int y, int w) {
        x = Find(x), y = Find(y);
        if (x == y) return false;
        if (g[x] > g[y]) swap(x, y);
        g[y] += g[x];
        f[x] = y;
        e[x] = w;
        return true;
    }
    int Size(int x) {
        return g[Find(x)];
    }
    int qry(int x, int y) {
        int r = 0;
        while (x != y) {
            if (g[x] > g[y]) swap(x, y);
            chmax(r, e[x]);
            x = f[x];
        }
        return r;
    }
};

void solve() {
    int n, m;
    cin >> n >> m;

    vector<array<int, 3>> edg(m);
    for (auto &[w, u, v] : edg) {
        cin >> u >> v >> w;
        u--, v--;
    }

    sort(all(edg));

    DSU dsu(n);
    
    vector<bool> use(m);
    for (int i = 0; i < m; i++) {
        auto [w, u, v] = edg[i];
        use[i] = dsu.Union(u, v, -1);
    }

    dsu = DSU(n);
    for (int i = 0; i < m; i++) {
        if (use[i]) continue;
        auto [w, u, v] = edg[i];
        dsu.Union(u, v, w);
    }

    if (dsu.Size(0) != n) {
        cout << "-1\n";
        return;
    }

    i64 ans = 0;
    for (int i = 0; i < m; i++) if (use[i]) {
        auto [w, u, v] = edg[i];
        ans += max(dsu.qry(u, v) + 1 - w, 0);
    }

    cout << ans << '\n';
}

signed main() {
    cin.tie(0)->sync_with_stdio(false);
    cin.exceptions(cin.failbit);
    int T = 1;
    // cin >> T;
    while (T--) {
        solve();
    }
    
    return 0;
}