// #pragma GCC target("sse,sse2,sse3,ssse3,sse4")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")

// hloya template v26
 
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#include <bits/stdc++.h>
using namespace std;

bool dbg = 0;
  
clock_t start_time = clock();
#define current_time fixed<<setprecision(6)<<(ld)(clock()-start_time)/CLOCKS_PER_SEC
  
#define f first
#define s second
#define mp make_pair
#define mt make_tuple
#define pb push_back
#define eb emplace_back
#define all(v) (v).begin(), (v).end()
#define sz(v) ((int)(v).size())
#define sqr(x) ((x) * (x))


#define ull unsigned long long
#define ll long long
#define ld long double
#define pii pair<int,int>
#define umap unordered_map<int, int>
  
#define files1 freopen("input.txt","r",stdin)
#define files2 freopen("blackjack.out","w",stdout)
#define files files1;files2
#define fast_io ios_base::sync_with_stdio(0);cin.tie(0)
  
#define endl '\n'
#define ln(i,n) " \n"[(i) == (n) - 1]
  
void bad(string mes = "Impossible"){cout << mes;exit(0);}
void bad(int mes){cout << mes;exit(0);}
  
template<typename T>
string bin(T x, int st = 2){
    string ans = "";
    while (x > 0){
        ans += char('0' + x % st);
        x /= st;
    }
    reverse(ans.begin(), ans.end());
    return ans.empty() ? "0" : ans;
}
  
mt19937_64 mt_rand(
    chrono::system_clock::now().time_since_epoch().count()
);

template<typename T1, typename T2> inline bool upmax(T1& a, T2 b) { return (a < b ? (a = b, true) : false); }
template<typename T1, typename T2> inline bool upmin(T1& a, T2 b) { return (b < a ? (a = b, true) : false); }

// inline int popcount(int x){
//     int count = 0;
//     __asm__ volatile("POPCNT %1, %0;":"=r"(count):"r"(x):);
//     return count;
// }
  
// template<typename T>
// T input(){
//     T ans = 0, m = 1;
//     char c = ' ';
  
//     while (!((c >= '0' && c <= '9') || c == '-')) {
//         c = getchar();
//     }
  
//     if (c == '-')
//         m = -1, c = getchar();
//     while (c >= '0' && c <= '9'){
//         ans = ans * 10 + (c - '0'), c = getchar();
//     }
//     return ans * m;
// }

template<typename T>
T gcd (T a, T b) { while (b) { a %= b; swap (a, b); } return a; }
  
// template<typename T> void read(T& a) { a = input<T>(); }
// template<typename T> void read(T& a, T& b) { read(a), read(b); }
// template<typename T> void read(T& a, T& b, T& c) { read(a, b), read(c); }
// template<typename T> void read(T& a, T& b, T& c, T& d) { read(a, b), read(c, d); }
  
const int inf = 1e9 + 20;
const short short_inf = 3e4 + 20;
const long double eps = 1e-12;
const int maxn = (int)1e5 + 3, base = (int) 1e9+2012;
const ll llinf = 2e18 + 5;

ll tree[4 * maxn];

void upd(int v, int tl, int tr, int pos, ll val) {
    if (tl == tr) {
        upmin(tree[v], val);
        return;
    }
    int tm = (tl + tr) >> 1;
    if (pos <= tm) {
        upd(v<<1, tl, tm, pos, val);
    } else {
        upd(v<<1|1, tm + 1, tr, pos, val);
    }
    tree[v] = min(tree[v<<1], tree[v<<1|1]);
}

ll get(int v, int tl,  int tr, int l, int r) {
    if (l > r) return llinf;
    if (tl == l && tr == r) {
        return tree[v];
    }
    int tm = (tl + tr) >> 1;
    return min(get(v<<1, tl, tm, l, min(r, tm)),  get(v<<1|1, tm + 1, tr, max(l, tm + 1), r));
}

int main() {
    // files1;
    fast_io;

    int n, m;
    cin >> n >> m;

    vector<tuple<int, int, int>> kek, mda;
    ll ans = llinf;

    for (int i = 0; i < 4 * maxn; i++) {
        tree[i] = llinf;
    }

    for (int i = 0; i < m; i++) {
        int l, r, c;
        cin >> l >> r >> c;

        if (l == 1 && r == n) {
            upmin(ans, c);
        } else if (l == 1) {
            upd(1, 1, n, r, c);
        } else {
            kek.eb(r, l, c);
        }
    }

    sort(all(kek));
    for (int i = 0; i < kek.size(); i++) {
        int r, l, c;
        tie(r, l, c) = kek[i];

        ll cc = get(1, 1, n, l, r);
        if (r == n) {
            upmin(ans, cc + c);
        }
        upd(1, 1, n, r, cc + c);
    }
    if (ans == llinf) {
        ans = -1;
    }
    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 5112