#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <tuple>
#include <iomanip>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
//typedef pair<ll, ll> P;
typedef vector<ll> vec;
typedef vector<vec> mat;
#define rep(i, n) for(ll i = 0; i < (n); i++)
#define revrep(i, n) for(ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
#define chmin(x, y) x = min(x, y);
#define chmax(x, y) x = max(x, y);
/*
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll max3(ll a, ll b, ll c){return max(a, max(b, c));};
ll min3(ll a, ll b, ll c){return min(a, min(b, c));};
ll max4(ll a, ll b, ll c, ll d){return max(max(a, b), min(c, d));};
ll min4(ll a, ll b, ll c, ll d){return min(min(a, b), min(c, d));};
ll max5(ll a, ll b, ll c, ll d, ll e){return max(max(a, b), max3(c, d, e));};
ll min5(ll a, ll b, ll c, ll d, ll e){return min(min(a, b), min3(c, d, e));};
*/
const ll INFL = 1LL << 60;//10^18 = 2^60
//const int INF = 1 << 30;//10^9
const ll INF = INFL;
ll MOD = 1000000007;
//ll MOD = 998244353;

vector<ll> dy = {0, -1, 0, 1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};

void pres(double A){cout << fixed << setprecision(10) << A << endl;}
//デバッグ
void BinarySay(ll x, ll y = 60){rep(i, y) cout << (x>>(y-1-i) & 1); cout << endl;}
ll get_bit(ll x){return __builtin_popcountll(x);}

ll pow_long(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2) res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}

ll pow_mod(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2){
      res *= x; res %= MOD;
    }
    x *= x; x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x){return pow_mod(x, MOD - 2);};

ll gcd(ll a, ll b){
    if(b == 0) return a;
    return gcd(b, a % b);
}

ll lcm(ll x, ll y){return x / gcd(x, y) * y;};

ll kai_mod(ll x){
  if(x == 0) return 1;
  return x * kai_mod(x-1) % MOD;
}

//約数の列挙O(√n)
vector<ll> divisor(ll n){
    vector<ll> res(0);
    for(ll i = 1; i * i <= n; i++){
        if(n % i == 0){
            res.push_back(i);
            if(i != n/i) res.push_back(n/i);
        }
    }
    sort(res.begin(), res.end());
    return res;
}

//素因数列挙
vector<ll> MakePrimeList(ll x) {
    vector<ll> res;
    for(ll k=2; k*k<=x; k++) {
        if(x % k == 0) {
            res.pb(k);
            while(x % k == 0) x /= k;
        }
    }
    if(x > 1) res.push_back(x);
    return res;
}


//コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
//facはn!,finvは1/n!
//invは逆元
void COMinit(){
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i = 2; i < MAXcomb; i++){
        fac[i] = fac[i-1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD/i) % MOD;
        finv[i] = finv[i-1] * inv[i] % MOD;
    }
}
ll comb(int n, int k){
    if(n < k) return 0;
    if(n < 0 || k < 0) return 0;
    return fac[n] * finv[k] % MOD * finv[n-k] % MOD;
}


//不可逆区間変更、区間最小
const ll N_ = 1<<18;
ll seg[2 * N_ - 1], lazy[2 * N_ - 1];

void init_seg(ll x = INFL){
  rep(i, 2 * N_ - 1){
    seg[i] = x;
    lazy[i] = x;
  }
}

void eval(ll k, ll l, ll r){
  chmin(seg[k], lazy[k]);
  if(r - l > 1){
    chmin(lazy[2 * k + 1], lazy[k]);
    chmin(lazy[2 * k + 2], lazy[k]);
  }
}

void update(ll a, ll b, ll x, ll k = 0, ll l = 0, ll r = N_){
  eval(k, l, r);
  if(r <= a || b <= l) return;
  if(a <= l && r <= b){
    chmin(lazy[k], x);
    eval(k, l, r);
    return;
  }
  update(a, b, x, 2 * k + 1, l, (l + r) / 2);
  update(a, b, x, 2 * k + 2, (l + r) / 2, r);
  seg[k] = min(seg[2 * k + 1], seg[2 * k + 2]);
  return;
}

ll getminimum(ll a, ll b, ll k = 0, ll l = 0, ll r = N_){
  if(r <= a || b <= l) return INFL;
  eval(k, l, r);
  if(a <= l && r <= b) return seg[k];
  ll res = INFL;
  res = min(res, getminimum(a, b, 2 * k + 1, l, (l + r) / 2));
  res = min(res, getminimum(a, b, 2 * k + 2, (l + r) / 2, r));
  return res;
}


int main(){
  ll N, M;
  cin >> N >> M;
  vector<pair<pair<ll, ll>, ll>> LRC(M);
  vector<ll> pos;
  pos.pb(0);
  pos.pb(N-1);
  rep(i, M){
    cin >> LRC[i].f.f >> LRC[i].f.s >> LRC[i].s;
    LRC[i].f.f--;
    LRC[i].f.s--;
    pos.pb(LRC[i].f.f);
  }
  sort(pos.begin(), pos.end());
  pos.erase(unique(pos.begin(), pos.end()), pos.end());
  sort(LRC.begin(), LRC.end());
  init_seg();
  update(0, 1, 0);
  ll s = 0;
  rep(i, pos.size()){
    ll x = getminimum(i, i+1);
    while(LRC[s].f.f == pos[i]){
      ll to = upper_bound(pos.begin(), pos.end(), LRC[s].f.s) - pos.begin();
      update(i, to, x + LRC[s].s);
      s++;
    }
  }
  
  ll ans = getminimum(pos.size()-1, pos.size());
  if(ans == INFL) cout << -1 << endl;
  else cout << ans << endl;
}


Measured memory_bytes: 13684