Problem: 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.

Examples

Input

4 3
1 3 2
2 4 3
1 4 6


Output

5


Input

4 2
1 2 1
3 4 2


Output

-1


Input

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


Output

28

Solution (in PYTHON3):

Solution:
from sys import stdout
import heapq
printn = lambda x: stdout.write(x)
inn = lambda : int(input())
inl   = lambda: list(map(int, input().split()))
inm   = lambda:      map(int, input().split())
DBG = True  and False
BIG = 999999999
R = 10**9 + 7

def ddprint(x):
  if DBG:
    print(x)

def dumpheap(h):
    a = []
    print("- -")
    while len(h)>0:
        z = heapq.heappop(h)
        print(z)
        a.append(z)
    for z in a:
        heapq.heappush(h,z)

n,m = inm()
a = []
for i in range(m):
    l,r,c = inm()
    a.append((l,r,c))
a.sort()
if a[0][0]>1:
    print(-1)
    exit()

ddprint(a)

h = []
top = 0
for i in range(m):
    l,r,c = a[i]
    if l==1:
        heapq.heappush(h,(c,0,l,r,c))
    else:
        top = i
        break

if DBG:
    dumpheap(h)

curld,curd,curl,curr,curc = heapq.heappop(h)
for i in range(top,m):
    l,r,c = a[i]
    while curr<l:
        if len(h)==0:
            print(-1)
            exit()
        curld,curd,curl,curr,curc = heapq.heappop(h)
    heapq.heappush(h,(curld,curd,curl,curr,curc))
    #ddprint("push {} {} {} {} {}".format(curld+c,curld,l,r,c))
    heapq.heappush(h,(curld+c,curld,l,r,c))

r=0
while r<n:
    if len(h)==0:
            print(-1)
            exit()
    ld,d,l,r,c = heapq.heappop(h)
print(ld)