#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
using Int = unsigned int;
using llong = long long;
using Llong = unsigned long long;
using ldouble = long double;
using intV = vector<int>;
using llongV = vector<long long>;
using llongVV = vector<vector<long long>>;
using Graph = vector<vector<long long>>;
using costGraph = vector<vector<pair<long long, long long>>>;
struct Edge {
  long long from;
  long long to;
  long long cost;
};
template <typename T>
using asc = less<T>();
template <typename T>
using desc = greater<T>();
const llong MOD = 1e9 + 7;
const llong INF = 1e17;
class SegmentTree {
 public:
  llongV __data;
  llong __size;
  SegmentTree(llong size) { this->init(size); };
  void init(llong size) {
    llong binSize = 0;
    while ((1 << binSize) < size) {
      binSize++;
    }
    this->__size = (1 << binSize);
    this->__data.resize(this->__size * 2);
    for (llong i = 0; i < this->__size * 2; i++) {
      this->__data[i] = INF;
    }
  };
  void update(llong index, llong value) {
    index += this->__size - 1;
    this->__data[index] = value;
    while (index > 0) {
      index = (index - 1) / 2;
      this->__data[index] =
          min(this->__data[index * 2 + 1], this->__data[index * 2 + 2]);
    }
  };
  llong __query(llong qLeft, llong qRight, llong node, llong left,
                llong right) {
    if (right <= qLeft || qRight <= left) return INF;
    if (qLeft <= left && right <= qRight) return this->__data[node];
    llong leftVal =
        this->__query(qLeft, qRight, node * 2 + 1, left, (left + right) / 2);
    llong rightVal =
        this->__query(qLeft, qRight, node * 2 + 2, (left + right) / 2, right);
    return min(leftVal, rightVal);
  }
  llong query(llong left, llong right) {
    return this->__query(left, right, 0, 0, this->__size);
  };
};
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  llong N, M;
  cin >> N >> M;
  SegmentTree tree(N + 1);
  tree.update(1, 0);
  vector<tuple<llong, llong, llong>> edges(M);
  for (llong i = 0LL; i < llong(M); i++)
    cin >> get<0>(edges[i]) >> get<1>(edges[i]) >> get<2>(edges[i]);
  sort(edges.begin(), edges.end());
  for (auto a : edges) {
    llong L, R, C;
    tie(L, R, C) = a;
    llong cost = tree.query(L, R + 1) + C;
    tree.update(R, min(cost, tree.query(R, R + 1)));
  }
  llong ans = tree.query(N, N + 1);
  if (ans % 2 == 0) ans = -1;
  if (ans == INF) ans = -1;
  cout << ans << endl;
  return 0;
}
