// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define repR(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i, n) for (int i = (int)(n); i >= 1; i--)
#define loop(i, a, B) for (int i = a; i B; i++)
#define loopR(i, a, B) for (int i = a; i B; i--)
#define all(x) begin(x), end(x)
#define allR(x) rbegin(x), rend(x)
#define rng(x, l, r) begin(x) + (l), begin(x) + (r)
#define pb push_back
#define eb emplace_back
#define fst first
#define snd second
template <class A, class B> constexpr auto mp(A &&a, B &&b) { return make_pair(forward<A>(a), forward<B>(b)); }
template <class... T> constexpr auto mt(T&&... x) { return make_tuple(forward<T>(x)...); }
template <class Int> auto constexpr inf_ = numeric_limits<Int>::max()/2-1;
auto constexpr INF32 = inf_<int32_t>;
auto constexpr INF64 = inf_<int64_t>;
auto constexpr INF   = inf_<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T, class Comp> struct pque : priority_queue<T, vector<T>, Comp> { vector<T> &data() { return this->c; } void clear() { this->c.clear(); } };
template <class T> using pque_max = pque<T, less<T>>;
template <class T> using pque_min = pque<T, greater<T>>;
template <class T, class = typename T::iterator, enable_if_t<!is_same<T, string>::value, int> = 0>
ostream& operator<<(ostream& os, T const& a) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, size_t N, enable_if_t<!is_same<T, char>::value, int> = 0>
ostream& operator<<(ostream& os, const T (&a)[N]) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, class = decltype(begin(declval<T&>())), class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &a) { for (auto& x : a) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T, S> const& p) { return os << p.first << " " << p.second; }
template <class T, class S> istream& operator>>(istream& is, pair<T, S>& p) { return is >> p.first >> p.second; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint { template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); } };
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T, d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T, d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T, 0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T, d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T, d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T, class U> constexpr bool chmin(T& x, U const& y) { if (x > (T)y) { x = (T)y; return true; } return false; }
template <class T, class U> constexpr bool chmax(T& x, U const& y) { if (x < (T)y) { x = (T)y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b, e, typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
template <class C, class T> int lbd(C const& v, T const& x) { return lower_bound(begin(v), end(v), x)-begin(v); }
template <class C, class T> int ubd(C const& v, T const& x) { return upper_bound(begin(v), end(v), x)-begin(v); }
constexpr int64_t mod(int64_t x, int64_t m) { assert(m > 0); return (x %= m) < 0 ? x+m : x; }
constexpr int64_t div_floor(int64_t x, int64_t y) { assert(y != 0); return x/y - ((x^y) < 0 and x%y); }
constexpr int64_t div_ceil(int64_t x, int64_t y) { assert(y != 0); return x/y + ((x^y) > 0 and x%y); }
constexpr int dx[] = { 1, 0, -1, 0, 1, -1, -1, 1 };
constexpr int dy[] = { 0, 1, 0, -1, 1, 1, -1, -1 };
constexpr int popcnt(ll x) { return __builtin_popcountll(x); }
mt19937_64 seed_{random_device{}()};
template <class Int> Int rand(Int a, Int b) { return uniform_int_distribution<Int>(a, b)(seed_); }
i64 irand(i64 a, i64 b) { return rand<i64>(a, b); } // [a, b]
u64 urand(u64 a, u64 b) { return rand<u64>(a, b); } //
template <class It> void shuffle(It l, It r) { shuffle(l, r, seed_); }
template <class T> vector<T> &operator--(vector<T> &v) { for (T &x : v) --x; return v; }
template <class T> vector<T> &operator++(vector<T> &v) { for (T &x : v) ++x; return v; }
// <<<
template <class Int> auto constexpr inf = numeric_limits<Int>::max()/2-1;
// >>> segment tree beats

template <class T, T max_id = -inf<T>, T min_id = inf<T>> struct SegtreeBeats {
    struct Data {
        T sum = 0, add = 0, max1 = 0, min1 = 0, max2 = max_id, min2 = min_id;
        int32_t maxc = 1, minc = 1;
    };
    vector<Data> v;
    int32_t n = 0, cap = 1;
    SegtreeBeats() {}
    template <class... U> SegtreeBeats(U&&... x) { init(forward<U>(x)...); }
    template <class F, class = decltype(declval<F>()(0))>
    void init(int n, F gen)  {
        cap = 1; while (cap < n) cap <<= 1;
        assert(n >= 0); this->n = n;
        v.assign(2*cap, {});
        for (int i = 0; i < n; i++) {
            v[cap+i].sum = v[cap+i].max1 = v[cap+i].min1 = gen(i);
        }
        for (int k = cap-1; k >= 1; k--) merge(k);
    }
    void init(int n, T x = 0) { init(n, [&](int) { return x; }); }
    void init(vector<T> const& v) { init(v.size(), [&](int i) { return v[i]; }); }
    int size() const { return n; }

    T operator[](int i) { return sum(i, i+1); }
    void set(int i, T x) {
        assert(0 <= i); assert(i < n);
        i += cap;
        for (int p = __lg(cap); p; --p) push(i >> p);
        v[i].sum = v[i].max1 = v[i].min1 = x;
        while (i >>= 1) merge(i);
    }
    void chmin(int l, int r, T x) {
        assert(0 <= l); assert(l <= r); assert(r <= n);
        chmin(l, r, x, 1, 0, cap);
    }
    void chmax(int l, int r, T x) {
        assert(0 <= l); assert(l <= r); assert(r <= n);
        chmax(l, r, x, 1, 0, cap);
    }
    void add(int l, int r, T x) {
        assert(0 <= l); assert(l <= r); assert(r <= n);
        query(l, r, [&](int k) { node_add(k, x); });
        for (l += cap, l >>= __builtin_ctzll(l); l >>= 1; ) merge(l);
        for (r += cap, r >>= __builtin_ctzll(r); r >>= 1; ) merge(r);
    }
    T sum(int l, int r) {
        assert(0 <= l); assert(l <= r); assert(r <= n);
        T x = 0; query(l, r, [&](int k) { x += v[k].sum; });
        return x;
    }
    T max(int l, int r) {
        assert(0 <= l); assert(l <= r); assert(r <= n);
        T x = max_id; query(l, r, [&](int k) { setmax(x, v[k].max1); });
        return x;
    }
    T min(int l, int r) {
        assert(0 <= l); assert(l <= r); assert(r <= n);
        T x = min_id; query(l, r, [&](int k) { setmin(x, v[k].min1); });
        return x;
    }
    vector<T> dat() {
        vector<T> ret(size());
        for (int i = 0; i < size(); i++) ret[i] = operator[](i);
        return ret;
    }

    static void setmin(T &x, T y) { x = std::min(x, y); }
    static void setmax(T &x, T y) { x = std::max(x, y); }
    void merge(int k) {
        if (k >= cap) return;
        const int L = k << 1, R = L | 1;
        v[k].sum = v[L].sum + v[R].sum;
        T d;

        d = v[L].min1 - v[R].min1;
        v[k].min1 = std::min(v[L].min1, v[R].min1);
        v[k].minc = (d <= 0 ? v[L].minc : 0) + (d >= 0 ? v[R].minc : 0);
        v[k].min2 = std::min(v[L].min2, v[R].min2);
        if (d != 0) setmin(v[k].min2, std::max(v[L].min1, v[R].min1));

        d = v[L].max1 - v[R].max1;
        v[k].max1 = std::max(v[L].max1, v[R].max1);
        v[k].maxc = (d >= 0 ? v[L].maxc : 0) + (d <= 0 ? v[R].maxc : 0);
        v[k].max2 = std::max(v[L].max2, v[R].max2);
        if (d != 0) setmax(v[k].max2, std::min(v[L].max1, v[R].max1));
    }
    void node_chmin(int k, T x) { // v[k].max2 < x < v[k].max1
        v[k].sum += (x-v[k].max1)*v[k].maxc;
        v[k].max1 = x;
        setmin(v[k].min1 > x ? v[k].min1 : v[k].min2, x);
    }
    void node_chmax(int k, T x) { // v[k].min2 > x > v[k].min1
        v[k].sum += (x-v[k].min1)*v[k].minc;
        v[k].min1 = x;
        setmax(v[k].max1 < x ? v[k].max1 : v[k].max2, x);
    }
    void node_add(int k, T x) {
        v[k].sum += x*(cap >> __lg(k)); v[k].add += x;
        v[k].max1 += x; v[k].max2 += x;
        v[k].min1 += x; v[k].min2 += x;
    }
    void push(int k) { // k < cap
        const int L = k << 1, R = L | 1;
        if (v[k].add) {
            node_add(L, v[k].add);
            node_add(R, v[k].add);
            v[k].add = 0;
        }
        if (v[k].max1 < v[L].max1) node_chmin(L, v[k].max1);
        if (v[k].max1 < v[R].max1) node_chmin(R, v[k].max1);
        if (v[k].min1 > v[L].min1) node_chmax(L, v[k].min1);
        if (v[k].min1 > v[R].min1) node_chmax(R, v[k].min1);
    }
    void chmin(int a, int b, T x, int k, int l, int r) {
        if (r <= a || b <= l || v[k].max1 <= x) return;
        if (a <= l && r <= b && v[k].max2 < x) {
            node_chmin(k, x);
        } else {
            push(k);
            chmin(a, b, x, k << 1, l, (l+r)/2);
            chmin(a, b, x, k << 1 | 1, (l+r)/2, r);
            merge(k);
        }
    }
    void chmax(int a, int b, T x, int k, int l, int r) {
        if (r <= a || b <= l || v[k].min1 >= x) return;
        if (a <= l && r <= b && v[k].min2 > x) {
            node_chmax(k, x);
        } else {
            push(k);
            chmax(a, b, x, k << 1, l, (l+r)/2);
            chmax(a, b, x, k << 1 | 1, (l+r)/2, r);
            merge(k);
        }
    }
    template <class F> void query(int l, int r, F f) {
        l += cap, r += cap;
        for (int p = __lg(cap); p; --p) push(l >> p), push((r-1) >> p);
        for ( ; l < r; l >>= 1, r >>= 1) {
            if (l&1) f(l++);
            if (r&1) f(--r);
        }
    }
};

// <<<

int32_t main() {
    int n, q; cin >> n >> q;
    vector<int> l(q), r(q), b(q);
    rep (i, q) cin >> l[i] >> r[i] >> b[i], --l[i];

    SegtreeBeats<int> seg(n, 1);
    rep (i, q) seg.chmax(l[i], r[i], b[i]);
    rep (i, q) if (seg.min(l[i], r[i]) != b[i]) quit(-1);

    cout << seg.dat() << '\n';


}