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 PyPy3 (2.4.0) (judge verdict: Time Limit Exceeded)

N,M = map(int,input().split())
LRC = [list(map(int,input().split())) for _ in range(M)]

def dijkstra(s,n,cost):
  #始点sから各頂点への最短距離
  #n:頂点数,　cost[u][v] : 辺uvのコスト(存在しないときはinf)
  d = [float("inf")] * n
  used = [False] * n
  d[s] = 0

  while True:
    v = -1
    #まだ使われてない頂点の中から最小の距離のものを探す
    for i in range(n):
      if (not used[i]) and (v == -1):
        v = i
      elif (not used[i]) and d[i] < d[v]:
        v = i
    if v == -1:
      break
    used[v] = True

    for j in range(n):
      d[j] = min(d[j],d[v]+cost[v][j])
  return d

INF = float('inf')
G = [[INF for _ in range(N)] for _ in range(N)]

for i in range(N-1):
  G[i+1][i] = 0
for l,r,c in LRC:
  l -= 1
  r -= 1
  G[l][r] = c
#print(G)

#print(dijkstra(0,N,G))
p = dijkstra(0,N,G)[N-1]
print(p if p != INF else -1)