#include <bits/stdc++.h>
//typedef
//-------------------------#include <bits/stdc++.h>

const double pi = 3.141592653589793238462643383279;

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s)
{
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T>
inline string toString(T x)
{
  ostringstream sout;
  sout << x;
  return sout.str();
}
inline int readInt()
{
  int x;
  scanf("%d", &x);
  return x;
}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util

//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())

//repetition
//------------------------------------------
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define MOD 1000000007

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()

typedef long long ll;
typedef pair<int, int> P;
typedef pair<int, int> pii;
typedef vector<int> vi;
const double EPS = 1E-8;

#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)

class UnionFind
{
public:
  vector<int> par;
  vector<int> siz;

  UnionFind(int sz_) : par(sz_), siz(sz_, 1)
  {
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(int sz_)
  {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }

  int root(int x)
  {
    if (par[x] == x)
    {
      return x;
    }
    else
    {
      return par[x] = root(par[x]);
    }
  }

  bool merge(int x, int y)
  {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(int x, int y)
  {
    return root(x) == root(y);
  }

  int size(int x)
  {
    return siz[root(x)];
  }
};

ll modPow(ll x, ll n, ll mod = MOD)
{
  if (n == 0)
    return 1;
  ll res = 1;
  while (n)
  {
    if (n & 1)
      res = (res * x) % mod;

    res %= mod;
    x = x * x % mod;
    n >>= 1;
  }
  return res;
}

#define SIEVE_SIZE 5000000 + 10
bool sieve[SIEVE_SIZE];
void makeSieve()
{
  for (int i = 0; i < SIEVE_SIZE; ++i)
    sieve[i] = true;
  sieve[0] = sieve[1] = false;
  for (int i = 2; i * i < SIEVE_SIZE; ++i)
    if (sieve[i])
      for (int j = 2; i * j < SIEVE_SIZE; ++j)
        sieve[i * j] = false;
}

bool isprime(ll n)
{
  if (n == 0 || n == 1)
    return false;
  for (ll i = 2; i * i <= n; ++i)
    if (n % i == 0)
      return false;
  return true;
}

const int MAX = 777778;
long long fac[MAX], finv[MAX], inv[MAX];

const ll mod2 = 998244353;
const ll mod3 = 1777777777;
// テーブルを作る前処理
void COMinit()
{
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++)
  {
    fac[i] = fac[i - 1] * i % mod3;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
    //cout << fac[i] << " " << inv[i] << " " << finv[i] << endl;
  }
}

// 二項係数計算
long long COM(int n, int k)
{
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

long long extGCD(long long a, long long b, long long &x, long long &y)
{
  if (b == 0)
  {
    x = 1;
    y = 0;
    return a;
  }
  long long d = extGCD(b, a % b, y, x);
  y -= a / b * x;
  return d;
}
// 負の数にも対応した mod (a = -11 とかでも OK)
inline long long mod(long long a, long long m)
{
  return (a % m + m) % m;
}

// 逆元計算 (ここでは a と m が互いに素であることが必要)
long long modinv(long long a, long long m)
{
  long long x, y;
  extGCD(a, m, x, y);
  return mod(x, m); // 気持ち的には x % m だが、x が負かもしれないので
}
ll GCD(ll a, ll b)
{

  if (b == 0)
    return a;
  return GCD(b, a % b);
}

typedef vector<ll> vec;
typedef vector<vec> mat;

mat mul(mat &A, mat &B)
{
  mat C(A.size(), vec((int)B[0].size()));
  for (int i = 0; i < A.size(); ++i)
  {
    for (int k = 0; k < B.size(); ++k)
    {
      for (int j = 0; j < B[0].size(); ++j)
      {
        C[i][j] = (C[i][j] + A[i][k] % MOD * B[k][j] % MOD) % MOD;
      }
    }
  }
  return C;
}
mat matPow(mat A, ll n)
{
  mat B(A.size(), vec((int)A.size()));

  for (int i = 0; i < A.size(); ++i)
  {
    B[i][i] = 1;
  }

  while (n > 0)
  {
    if (n & 1)
      B = mul(B, A);
    A = mul(A, A);
    n >>= 1;
  }
  return B;
}

template <class TYPE>
struct Fenwick
{
  int N;
  vector<ll> bit;

  Fenwick() : N(0) {}
  Fenwick(int n) : N(n), bit(n + 1, 0) {}

  void add(int i, ll x)
  {
    //0-indexedを避けるために加算
    while (i <= N)
    {
      bit[i] += x;
      i += (i & -i);
    }
  }

  ll sum(int i)
  {
    ll res = 0;
    while (i > 0)
    {
      res += bit[i];
      i -= (i & -i);
    }
    return res;
  }

  ll range(int i, int j)
  {
    return sum(j) - sum(i);
  }

  ll get(int i)
  {
    return sum(i) - sum(i - 1);
  }

  ll getKth(ll k)
  {
    ++k;
    int res = 0;
    int n = 1;
    while (n < bit.size())
      n *= 2;

    for (int i = n / 2; i > 0; i /= 2)
    {
      if (res + i < bit.size() && bit[res + i] < k)
      {
        k = k - bit[res + i];
        res = res + i;
      }
    }
    return res + 1;
  }

  // bitは1-indexedで実装しているのでバグった時は初めに確認すること
};

struct LazySegmentTree
{
private:
  int n;
  vector<ll> node, lazy;
  vector<bool> lazyFlag;

public:
  LazySegmentTree(vector<ll> v)
  {
    int sz = (int)v.size();
    n = 1;
    while (n < sz)
      n *= 2;
    node.resize(2 * n - 1);
    lazy.resize(2 * n - 1, (1LL << 60));
    lazyFlag.resize(2 * n - 1, false);

    for (int i = 0; i < sz; i++)
      node[i + n - 1] = v[i];
    for (int i = n - 2; i >= 0; i--)
      node[i] = min(node[i * 2 + 1], node[i * 2 + 2]);
  }

  void lazyEvaluate(int k, int l, int r)
  {
    if (lazyFlag[k])
    {
      node[k] = lazy[k];
      if (r - l > 1)
      {
        lazy[k * 2 + 1] = lazy[k * 2 + 2] = lazy[k];
        lazyFlag[k * 2 + 1] = lazyFlag[k * 2 + 2] = true;
      }
      lazyFlag[k] = false;
    }
  }

  void update(int a, int b, ll x, int k = 0, int l = 0, int r = -1)
  {
    if (r < 0)
      r = n;
    lazyEvaluate(k, l, r);
    if (b <= l || r <= a)
      return;
    if (a <= l && r <= b)
    {
      lazy[k] = x;
      lazyFlag[k] = true;
      lazyEvaluate(k, l, r);
    }
    else
    {
      update(a, b, x, 2 * k + 1, l, (l + r) / 2);
      update(a, b, x, 2 * k + 2, (l + r) / 2, r);
      node[k] = min(node[2 * k + 1], node[2 * k + 2]);
    }
  }

  ll find(int a, int b, int k = 0, int l = 0, int r = -1)
  {
    if (r < 0)
      r = n;
    lazyEvaluate(k, l, r);
    if (b <= l || r <= a)
      return (1LL << 60);
    if (a <= l && r <= b)
      return node[k];
    ll vl = find(a, b, 2 * k + 1, l, (l + r) / 2);
    ll vr = find(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};
ll N, M;

//iにいるときの最小コスト
struct SegmentTree
{
private:
  int n;
  vector<ll> node;

public:
  SegmentTree(vector<ll> v)
  {
    int sz = v.size();
    n = 1;
    while (n < sz)
      n *= 2;
    node.resize(2 * n - 1, (1LL << 60));
    for (int i = 0; i < sz; i++)
      node[i + n - 1] = v[i];
    for (int i = n - 2; i >= 0; i--)
      node[i] = min(node[2 * i + 1], node[2 * i + 2]);
  }

  void update(ll x, ll val)
  {
    x += (n - 1);
    node[x] = val;
    while (x > 0)
    {
      x = (x - 1) / 2;
      node[x] = min(node[2 * x + 1], node[2 * x + 2]);
    }
  }

  ll getmin(int a, int b, int k = 0, int l = 0, int r = -1)
  {
    if (r < 0)
      r = n;
    if (r <= a || b <= l)
      return (1LL << 60);
    if (a <= l && r <= b)
      return node[k];

    ll vl = getmin(a, b, 2 * k + 1, l, (l + r) / 2);
    ll vr = getmin(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};
int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(4);

  cin >> N >> M;

  vector<pair<pair<int, int>, ll>> vp;
  REP(i, M)
  {
    int L, R;

    ll C;
    cin >> L >> R >> C;
    L--;
    R--;
    vp.push_back(make_pair(make_pair(L, R), C));
  }

  sort(all(vp));

  vector<ll> dp(N, (1LL << 60));

  dp[0] = 0;
  SegmentTree seg(dp);
  for (int i = 0; i < M; i++)
  {
    int s = vp[i].first.first, t = vp[i].first.second;
    ll cost = vp[i].second;

    ll tmp = seg.getmin(s, t + 1);
    dp[t] = min(dp[t], tmp + cost);
    seg.update(t, dp[t]);
  }
  if (seg.getmin(N - 1, N) == (1LL << 60))
    cout << -1 << endl;
  else
    cout << seg.getmin(N - 1, N) << endl;
  return 0;
}

Measured memory_bytes: 5360