import std.stdio;
import std.string;
import std.format;
import std.conv;
import std.typecons;
import std.algorithm;
import std.functional;
import std.bigint;
import std.numeric;
import std.array;
import std.math;
import std.range;
import std.container;
import std.concurrency;
import std.traits;
import std.uni;
import core.bitop : popcnt;
alias Generator = std.concurrency.Generator;

enum long INF = long.max/5;
enum long MOD = 10L^^9+7;

void main() {
  long N, M;
  scanln(N, M);

  auto dij = Dijkstra!long(N);
  foreach(i; 0..N-1) {
    dij.addEdge(i+1, i, 0);
  }
  foreach(_; 0..M) {
    long l, r, c;
    scanln(l, r, c);
    l--; r--;
    dij.addEdge(l, r, c);
  }
  dij.solve(0).back.pipe!(a => a>=INF?-1L:a).writeln;
}

// Dijkstra's algorithm
// O((V+E)logV)
struct Dijkstra(T) {
  import std.algorithm;
  import std.container; // rbtree
private:
  Vertex[] _vertices;

public:
  this(size_t size, T inf = T.max) {
    _vertices.length = size;
    foreach(i; 0..size) {
      _vertices[i] = new Vertex(i, inf);
    }
  }

  void addEdge(size_t start, size_t end, T weight) {
    _vertices[start].edges ~= Edge(start, end, weight);
  }

  void addEdge(T[] input) {
    addEdge(cast(size_t) input[0], cast(size_t) input[1], input[2]);
  }

  T[] solve(size_t start, size_t end = -1) {
    _vertices[start].cost = 0;
    auto rbtree = redBlackTree!"a.cost==b.cost ? a.index<b.index : a.cost<b.cost"(_vertices[start]);
    while(!rbtree.empty) {
      Vertex v = rbtree.front;
      if (v.index == end) break;
      v.flag = true;
      rbtree.removeFront;
      v.edges.each!((Edge e) {
        if (_vertices[e.end].flag) return;
        if (v.cost+e.weight < _vertices[e.end].cost) {
          rbtree.removeKey(_vertices[e.end]);
          _vertices[e.end].cost = v.cost+e.weight;
          rbtree.insert(_vertices[e.end]);
        }
      });
    }
    return _vertices.map!(v => v.cost).array;
  }

  Vertex[] getVertices() {
    return _vertices;
  }

private:
  class Vertex {
    size_t index;
    bool flag;
    T cost;
    Edge[] edges = [];
    this(size_t index, T cost) {
      this.index = index;
      this.cost = cost;
    }
    override string toString() const {return "";} // for old compilers
  }
  struct Edge {
    size_t start;
    size_t end;
    T weight;
  }
}


// ----------------------------------------------


void times(alias fun)(long n) {
  // n.iota.each!(i => fun());
  foreach(i; 0..n) fun();
}
auto rep(alias fun, T = typeof(fun()))(long n) {
  // return n.iota.map!(i => fun()).array;
  T[] res = new T[n];
  foreach(ref e; res) e = fun();
  return res;
}

T ceil(T)(T x, T y) if (isIntegral!T || is(T == BigInt)) {
  // `(x+y-1)/y` will only work for positive numbers ...
  T t = x / y;
  if (t * y < x) t++;
  return t;
}

T floor(T)(T x, T y) if (isIntegral!T || is(T == BigInt)) {
  T t = x / y;
  if (t * y > x) t--;
  return t;
}

ref T ch(alias fun, T, S...)(ref T lhs, S rhs) {
  return lhs = fun(lhs, rhs);
}
unittest {
  long x = 1000;
  x.ch!min(2000);
  assert(x == 1000);
  x.ch!min(3, 2, 1);
  assert(x == 1);
  x.ch!max(100).ch!min(1000); // clamp
  assert(x == 100);
  x.ch!max(0).ch!min(10); // clamp
  assert(x == 10);
}

mixin template Constructor() {
  import std.traits : FieldNameTuple;
  this(Args...)(Args args) {
    // static foreach(i, v; args) {
    foreach(i, v; args) {
      mixin("this." ~ FieldNameTuple!(typeof(this))[i]) = v;
    }
  }
}

void scanln(Args...)(auto ref Args args) {
  enum sep = " ";
  enum n = Args.length;
  enum fmt = n.rep!(()=>"%s").join(sep) ~ "\n";
  static if (__VERSION__ >= 2071) {
    readf!fmt(args);
  } else {
    enum argsTemp = n.iota.map!(
      i => "&args[%d]".format(i)
    ).join(", ");
    mixin(
      "readf(fmt, " ~ argsTemp ~ ");"
    );
  }
}

// fold was added in D 2.071.0
static if (__VERSION__ < 2071) {
  template fold(fun...) if (fun.length >= 1) {
    auto fold(R, S...)(R r, S seed) {
      static if (S.length < 2) {
        return reduce!fun(seed, r);
      } else {
        return reduce!fun(tuple(seed), r);
      }
    }
  }
}

// popcnt with ulongs was added in D 2.071.0
static if (__VERSION__ < 2071) {
  ulong popcnt(ulong x) {
    x = (x & 0x5555555555555555L) + (x>> 1 & 0x5555555555555555L);
    x = (x & 0x3333333333333333L) + (x>> 2 & 0x3333333333333333L);
    x = (x & 0x0f0f0f0f0f0f0f0fL) + (x>> 4 & 0x0f0f0f0f0f0f0f0fL);
    x = (x & 0x00ff00ff00ff00ffL) + (x>> 8 & 0x00ff00ff00ff00ffL);
    x = (x & 0x0000ffff0000ffffL) + (x>>16 & 0x0000ffff0000ffffL);
    x = (x & 0x00000000ffffffffL) + (x>>32 & 0x00000000ffffffffL);
    return x;
  }
}


Measured memory_bytes: 29436