#include <bits/stdc++.h>
using namespace std;
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
inline long long toLongLong(string s) {
  long long v;
  istringstream sin(s);
  sin >> v;
  return v;
}
__int128 toLint(const string s) {
  __int128 ret = 0;
  for (int i = 0; i < s.length(); i++)
    if ('0' <= s[i] && s[i] <= '9') ret = 10 * ret + s[i] - '0';
  return ret;
}
template <class T>
inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
inline vector<char> toVC(string s) {
  vector<char> data(s.begin(), s.end());
  return data;
}
vector<string> SPRIT(const string &s, const string &delim) {
  vector<string> result;
  string::size_type pos = 0;
  while (pos != string::npos) {
    string::size_type p = s.find(delim, pos);
    if (p == string::npos) {
      result.push_back(s.substr(pos));
      break;
    } else {
      result.push_back(s.substr(pos, p - pos));
    }
    pos = p + delim.size();
  }
  return result;
}
string TRIM(const string &str, const char *trimCharacterList = " \t\v\r\n") {
  string result;
  string::size_type left = str.find_first_not_of(trimCharacterList);
  if (left != string::npos) {
    string::size_type right = str.find_last_not_of(trimCharacterList);
    result = str.substr(left, right - left + 1);
  }
  return result;
}
string REPLACE_STRING(const string source, const string find,
                      const string alt) {
  string result = source;
  string::size_type pos = 0;
  while (pos = result.find(find, pos), pos != string::npos) {
    result.replace(pos, find.length(), alt);
    pos += alt.length();
  }
  return result;
}
template <typename T>
bool VECTOR_EXISTS(vector<T> vec, T data) {
  auto itr = std::find(vec.begin(), vec.end(), data);
  size_t index = distance(vec.begin(), itr);
  if (index != vec.size())
    return true;
  else
    return false;
}
template <typename T>
void VECTOR_REMOVE_VALUE_ALL(vector<T> &vec, T data) {
  vec.erase(remove(vec.begin(), vec.end(), data), vec.end());
}
template <typename T>
vector<T> VECTOR_REMOVE_VALUE_ALL_FAST(vector<T> vec, T data) {
  vector<T> vec2;
  for (auto &x : vec)
    if (x != data) vec2.push_back(x);
  return vec2;
}
bool REG_MATCH(string const &text, regex const &re) {
  bool result = regex_match(text, re);
  return result;
}
bool REG_MATCH(string const &text, smatch &match, regex const &re) {
  bool result = regex_match(text, match, re);
  return result;
}
smatch REG_SEARCH(string const &text, regex const &re) {
  smatch m;
  regex_search(text, m, re);
  return m;
}
vector<smatch> REG_ALL_SEARCH(string const &text, regex const &re) {
  vector<smatch> matchs;
  sregex_iterator iter(text.cbegin(), text.cend(), re);
  sregex_iterator end;
  for (; iter != end; iter++) matchs.push_back(*iter);
  return matchs;
}
string REG_REPLACE(string const &text, regex const &re, string const &replace) {
  string result =
      regex_replace(text, re, replace, regex_constants::format_first_only);
  return result;
}
string REG_ALL_REPLACE(string const &text, regex const &re,
                       string const &replace) {
  string result = regex_replace(text, re, replace);
  return result;
}
template <typename T>
vector<T> COMPRESS(vector<T> v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
  return v;
}
template <typename T>
pair<map<T, int>, map<int, T>> MAPPING(const vector<T> v) {
  map<T, int> zip;
  map<int, T> unzip;
  for (int i = 0; i < v.size(); i++) zip[v[i]] = i;
  for (int i = 0; i < v.size(); i++) unzip[i] = v[i];
  return make_pair(zip, unzip);
}
template <typename T>
vector<pair<T, long long>> RUN_LENGTH(const vector<T> &v) {
  vector<pair<T, long long>> ret;
  int len = v.size();
  for (int i = 0, j = 0; i < len; i = j) {
    while (j < len && v[i] == v[j]) j++;
    ret.push_back(make_pair(v[i], j - i));
  }
  return ret;
}
template <typename T, typename U, typename V, typename W>
auto operator+(const std::pair<T, U> &l, const std::pair<V, W> &r)
    -> std::pair<decltype(l.first + r.first), decltype(l.second + r.second)> {
  return {l.first + r.first, l.second + r.second};
}
template <typename T, typename U, typename V, typename W>
auto operator-(const std::pair<T, U> &l, const std::pair<V, W> &r)
    -> std::pair<decltype(l.first - r.first), decltype(l.second - r.second)> {
  return {l.first - r.first, l.second - r.second};
}
ostream &operator<<(ostream &dest, __int128_t value) {
  ostream::sentry s(dest);
  if (s) {
    __uint128_t tmp = value < 0 ? -value : value;
    char buffer[128];
    char *d = std::end(buffer);
    do {
      --d;
      *d = "0123456789"[tmp % 10];
      tmp /= 10;
    } while (tmp != 0);
    if (value < 0) {
      --d;
      *d = '-';
    }
    int len = std::end(buffer) - d;
    if (dest.rdbuf()->sputn(d, len) != len) {
      dest.setstate(ios_base::badbit);
    }
  }
  return dest;
}
double ceil_n(double dIn, long long nLen) {
  double dOut;
  dOut = dIn * pow(10.0, nLen);
  dOut = (double)(long long)(dOut + 0.9);
  return dOut * pow(10.0, -nLen);
}
double floor_n(double dIn, long long nLen) {
  double dOut;
  dOut = dIn * pow(10.0, nLen);
  dOut = (double)(long long)(dOut);
  return dOut * pow(10.0, -nLen);
}
double round_n(double dIn, long long nLen) {
  double dOut;
  dOut = dIn * pow(10.0, nLen);
  dOut = (double)(long long)(dOut + 0.5);
  return dOut * pow(10.0, -nLen);
}
int take_a_n(long long num, int n) {
  string str = toString(num);
  return str[str.length() - n] - '0';
}
int bitcount32(int bits) {
  bits = (bits & 0x55555555) + (bits >> 1 & 0x55555555);
  bits = (bits & 0x33333333) + (bits >> 2 & 0x33333333);
  bits = (bits & 0x0f0f0f0f) + (bits >> 4 & 0x0f0f0f0f);
  bits = (bits & 0x00ff00ff) + (bits >> 8 & 0x00ff00ff);
  return (bits & 0x0000ffff) + (bits >> 16 & 0x0000ffff);
}
long long bitcount64(long long bits) {
  bits = (bits & 0x5555555555555555) + (bits >> 1 & 0x5555555555555555);
  bits = (bits & 0x3333333333333333) + (bits >> 2 & 0x3333333333333333);
  bits = (bits & 0x0f0f0f0f0f0f0f0f) + (bits >> 4 & 0x0f0f0f0f0f0f0f0f);
  bits = (bits & 0x00ff00ff00ff00ff) + (bits >> 8 & 0x00ff00ff00ff00ff);
  bits = (bits & 0x0000ffff0000ffff) + (bits >> 16 & 0x0000ffff0000ffff);
  return (bits & 0x00000000ffffffff) + (bits >> 32 & 0x00000000ffffffff);
}
template <class T1, class T2>
inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T1, class T2>
inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
  in >> p.first >> p.second;
  return in;
}
template <typename T>
istream &operator>>(istream &in, vector<T> &v) {
  for (auto &x : v) in >> x;
  return in;
}
istream &operator>>(istream &in, __int128 &d) {
  string s;
  cin >> s;
  __int128 tmp = 0;
  for (int i = 0; i < s.length(); i++)
    if ('0' <= s[i] && s[i] <= '9') tmp = 10 * tmp + s[i] - '0';
  d = tmp;
  return in;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) {
  out << "[" << p.first << ", " << p.second << "]"
      << "\n";
  return out;
}
const double EPS = 1e-10;
const double PI = acos(-1.0);
template <typename T>
inline bool BETWEEN(const T aim, const T min, const T max) {
  if (min <= aim && aim <= max) {
    return true;
  } else {
    return false;
  }
}
template <typename T1, typename T2>
inline bool IS_OUT(const T1 toY, const T1 toX, const T2 H, const T2 W) {
  return (toY < 0 || toY >= H || toX < 0 || toX >= W);
}
template <class T>
inline T SQR(const T x) {
  return x * x;
}
template <class T1, class T2>
inline T1 POW(const T1 x, const T2 y) {
  if (!y)
    return 1;
  else if ((y & 1) == 0) {
    return SQR(POW(x, y >> 1));
  } else
    return POW(x, y ^ 1) * x;
}
template <typename T>
constexpr T ABS(T x) {
  return x < 0 ? -x : x;
}
template <class Iter>
bool next_partial_permutation(Iter first, Iter middle, Iter last) {
  reverse(middle, last);
  return next_permutation(first, last);
}
template <class Iter>
bool next_combination(Iter first1, Iter last1, Iter first2, Iter last2) {
  if ((first1 == last1) || (first2 == last2)) {
    return false;
  }
  Iter m1 = last1;
  Iter m2 = last2;
  --m2;
  while (--m1 != first1 && !(*m1 < *m2)) {
  }
  bool result = (m1 == first1) && !(*first1 < *m2);
  if (!result) {
    while (first2 != m2 && !(*m1 < *first2)) {
      ++first2;
    }
    first1 = m1;
    std::iter_swap(first1, first2);
    ++first1;
    ++first2;
  }
  if ((first1 != last1) && (first2 != last2)) {
    m1 = last1;
    m2 = first2;
    while ((m1 != first1) && (m2 != last2)) {
      std::iter_swap(--m1, m2);
      ++m2;
    }
    std::reverse(first1, m1);
    std::reverse(first1, last1);
    std::reverse(m2, last2);
    std::reverse(first2, last2);
  }
  return !result;
}
template <typename T>
constexpr bool ODD(T x) {
  return x % 2 != 0;
}
template <typename T>
constexpr bool EVEN(T x) {
  return x % 2 == 0;
}
template <class T>
inline T GCD(const T x, const T y) {
  if (x < 0) return GCD(-x, y);
  if (y < 0) return GCD(x, -y);
  return (!y) ? x : GCD(y, x % y);
}
template <class T>
inline T LCM(const T x, const T y) {
  if (x < 0) return LCM(-x, y);
  if (y < 0) return LCM(x, -y);
  return x * (y / GCD(x, y));
}
template <class T>
inline T EXTGCD(const T a, const T b, T &x, T &y) {
  if (a < 0) {
    T d = EXTGCD(-a, b, x, y);
    x = -x;
    return d;
  }
  if (b < 0) {
    T d = EXTGCD(a, -b, x, y);
    y = -y;
    return d;
  }
  if (!b) {
    x = 1;
    y = 0;
    return a;
  } else {
    T d = EXTGCD(b, a % b, x, y);
    T t = x;
    x = y;
    y = t - (a / b) * y;
    return d;
  }
}
template <class T>
inline T INV_MOD(const T a, const T m) {
  T x, y;
  EXTGCD(a, m, x, y);
  return (x + m) % m;
}
long long POW_MOD(long long N, long long P, long long M) {
  long long ret = 1;
  while (P > 0) {
    if (P & 1LL) ret = (ret * N) % M;
    N = (N * N) % M;
    P >>= 1LL;
  }
  return ret;
}
long long ARITH_SUM(long long a, long long d, long long n) {
  return (2 * a + (n - 1) * d) * n / 2;
}
long long ARITH_SUM_MOD(long long a, long long d, long long n, long long MOD) {
  return (2 * a + (n - 1) * d) % MOD * n % MOD * POW_MOD(2, MOD - 2, MOD) % MOD;
}
template <class T>
inline bool ISPRIME(const T x) {
  if (x <= 1) return false;
  for (T i = 2; i * i <= x; i++)
    if (x % i == 0) return false;
  return true;
}
template <class T>
vector<bool> ERATOSTHENES(const T n) {
  vector<bool> arr(n + 1, true);
  arr[0] = arr[1] = false;
  for (T i = 2; i * i <= n; i++) {
    if (arr[i]) {
      for (T j = i * 2LL; j <= n; j += i) {
        arr[j] = false;
      }
    }
  }
  return arr;
}
template <typename T>
vector<bool> ERATOSTHENES(const T a, const T b) {
  vector<bool> small = ERATOSTHENES(b);
  vector<bool> prime(b - a, true);
  for (T i = 2; i * i <= b; i++) {
    if (small[i]) {
      for (T j = max(2LL, (a + i - 1) / i) * i; j < b; j += i) {
        prime[j - a] = false;
      }
    }
  }
  return prime;
}
template <typename T>
vector<T> DIVISOR(T n) {
  vector<T> v;
  for (T i = 1; i * i <= n; ++i) {
    if (n % i == 0) {
      v.push_back(i);
      if (i != n / i) {
        v.push_back(n / i);
      }
    }
  }
  sort(v.begin(), v.end());
  return v;
}
template <typename T>
vector<vector<T>> DIVISOR_ALL(T n) {
  vector<vector<T>> res(n + 1);
  for (T i = 1; i <= n; i++) {
    for (T j = i; j <= n; j += i) {
      res[j].push_back(i);
    }
  }
  return res;
}
template <typename T>
vector<pair<T, T>> FACTORIZATION(T x) {
  vector<pair<T, long long>> ans;
  for (T i = 2; i * i <= x; i++) {
    if (x % i == 0) {
      T count = 0;
      while (x % i == 0) {
        count++;
        x /= i;
      }
      ans.push_back(make_pair(i, count));
    }
  }
  if (x != 1) ans.push_back(make_pair(x, 1));
  return ans;
}
template <typename T>
inline T NCR(T n, T r) {
  if (r > n - r) r = n - r;
  T ans = 1LL;
  for (T i = 0; i < r; i++) {
    ans = ans * (n - i) / (i + 1);
  }
  return ans;
}
long long NCR_MOD(long long n, long long r, long long M) {
  if (r > n - r) return NCR_MOD(n, n - r, M);
  long long numerator = 1LL;
  long long denominator = 1LL;
  for (long long i = 0; i < r; i++) {
    numerator *= (n - i);
    numerator %= M;
    denominator *= (i + 1);
    denominator %= M;
  }
  return numerator * POW_MOD(denominator, M - 2, M) % M;
}
const long long INF = 1e16;
struct LazySegmentTree {
 private:
  long long n;
  vector<long long> node, lazy;
  vector<bool> lazyFlag;

 public:
  LazySegmentTree(vector<long long> v) {
    long long sz = (long long)v.size();
    n = 1;
    while (n < sz) n *= 2;
    node.resize(2 * n - 1);
    lazy.resize(2 * n - 1, INF);
    lazyFlag.resize(2 * n - 1, false);
    for (long long i = 0; i < sz; i++) node[i + n - 1] = v[i];
    for (long long i = n - 2; i >= 0; i--)
      node[i] = min(node[i * 2 + 1], node[i * 2 + 2]);
  }
  void lazyEvaluate(long long k, long long l, long long r) {
    if (lazyFlag[k]) {
      node[k] = min(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(long long a, long long b, long long x, long long k = 0,
              long long l = 0, long long r = -1) {
    if (r < 0) r = n;
    lazyEvaluate(k, l, r);
    if (b <= l || r <= a) return;
    if (a <= l && r <= b) {
      lazy[k] = min(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]);
    }
  }
  long long find(long long a, long long b, long long k = 0, long long l = 0,
                 long long r = -1) {
    if (r < 0) r = n;
    lazyEvaluate(k, l, r);
    if (b <= l || r <= a) return INF;
    if (a <= l && r <= b) return node[k];
    long long vl = find(a, b, 2 * k + 1, l, (l + r) / 2);
    long long vr = find(a, b, 2 * k + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
};
int main() {
  long long N, M;
  cin >> N >> M;
  LazySegmentTree tree(vector<long long>(N, INF));
  vector<tuple<long long, long long, long long>> edges;
  vector<long long> L(M), R(M), C(M);
  for (int i = (0); i < (M); ++i) {
    cin >> L[i] >> R[i] >> C[i];
    L[i]--, R[i]--;
    edges.push_back(make_tuple(L[i], R[i], C[i]));
  }
  sort((edges).begin(), (edges).end());
  tree.update(0, 1, 0);
  for (int i = 0; i < M; i++) {
    long long l = get<0>(edges[i]);
    long long r = get<1>(edges[i]);
    long long cost = get<2>(edges[i]);
    long long s = tree.find(l, l + 1);
    if (s >= INF) continue;
    long long toCost = s + cost;
    tree.update(l + 1, r + 1, toCost);
  }
  long long ans = tree.find(N - 1, N);
  if (ans >= INF)
    cout << -1 << endl;
  else
    cout << ans << endl;
  return 0;
}
