Problem:
Score : 600 points

Problem Statement

We have N points numbered 1 to N arranged in a line in this order.

Takahashi decides to make an undirected graph, using these points as the vertices.
In the beginning, the graph has no edge. Takahashi will do M operations to add edges in this graph.
The i-th operation is as follows:

The operation uses integers L_i and R_i between 1 and N (inclusive), and a positive integer C_i. For every pair of integers (s, t) such that L_i \leq s < t \leq R_i, add an edge of length C_i between Vertex s and Vertex t.

The integers L_1, ..., L_M, R_1, ..., R_M, C_1, ..., C_M are all given as input.

Takahashi wants to solve the shortest path problem in the final graph obtained. Find the length of the shortest path from Vertex 1 to Vertex N in the final graph.

Constraints

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq L_i < R_i \leq N

1 \leq C_i \leq 10^9

Input

Input is given from Standard Input in the following format:

N M
L_1 R_1 C_1
:
L_M R_M C_M

Output

Print the length of the shortest path from Vertex 1 to Vertex N in the final graph.
If there is no shortest path, print -1 instead.

Sample Input 1

4 3
1 3 2
2 4 3
1 4 6

Sample Output 1

5

We have an edge of length 2 between Vertex 1 and Vertex 2, and an edge of length 3 between Vertex 2 and Vertex 4, so there is a path of length 5 between Vertex 1 and Vertex 4.

Sample Input 2

4 2
1 2 1
3 4 2

Sample Output 2

-1

Sample Input 3

10 7
1 5 18
3 4 8
1 3 5
4 7 10
5 9 8
6 10 5
8 10 3

Sample Output 3

28

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using vb = vector<bool>;
using vl = vector<long>;
using vs = vector<string>;
using vvi = vector<vector<int>>;
using vvb = vector<vector<bool>>;
using vvc = vector<vector<char>>;
using vvl = vector<vector<long>>;
using pii = pair<int, int>;
using pil = pair<int, long>;
using pll = pair<long, long>;
#define fix20 cout << fixed << setprecision(20)
#define YES cout << "Yes" << endl
#define NO cout << "No" << endl
#define rep(i,n) for(int i=0; i<(int)(n);i++)
#define REP(i,s,t) for(int i=s; i<t; i++)
#define RNG(i,s,t,u) for(int i=s; i<t; i+=u)
#define MOD 1000000007
#define all(vec) vec.begin(), vec.end()

struct Node{
  int now;
  //int sub;
  long cost;
};

struct Edge{
  int from;
  int to;
  long cost;
};

bool operator<(const Node &a, const Node &b){
  if(a.cost != b.cost){
    return a.cost < b.cost;
  }else{
    return a.now < b.now;
  }
}
bool operator>(const Node &a, const Node &b){return (b < a);}
bool operator<=(const Node &a, const Node &b){return !(b < a);}
bool operator>=(const Node &a, const Node &b){return !(a < b);}

//vector<vector<Edge>> Graph(n);
//sは始点
vector<long> Dijkstra(vector<vector<Edge>> &Graph, int s){ //最小コスト
  priority_queue<Node, vector<Node>, greater<Node>> que;
  vector<long> dist(Graph.size(), 1e16);

  dist.at(s) = 0;
  que.push(Node{s, 0});

  while(que.size()){
    Node node = que.top();
    que.pop();
    int v = node.now;
    if(node.cost > dist.at(v)) continue;
    for(Edge edge : Graph.at(v)){
      int nv = edge.to;
      long cost = edge.cost;
      if(dist.at(nv) > dist.at(v) + cost){
        dist.at(nv) = dist.at(v) + cost;
        que.push(Node{nv, dist.at(nv)});
      }
    }
  }
  return dist;
}

int main(){
  long n,m;
  cin >> n >> m;
  vector<vector<Edge>> Graph(n);
  REP(i,1,n){
    Graph.at(i).push_back(Edge{i,i-1,0});
  }
  rep(i,m){
    long l,r,c;
    cin >> l >> r >> c;
    l--;r--;
    Graph.at(l).push_back(Edge{l,r,c});
  }
  vector<long> dist = Dijkstra(Graph,0);
  if(dist.at(n-1) > 1e15) cout << -1 << endl;
  else cout << dist.at(n-1) << endl;
}