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:
def func(x,y):
    return min(x,y)

class SegmentTree:
    def __init__(self, n): #a1, a2, ..., an
        n_ = 1
        while n_ < n:
            n_ *= 2

        self.n = n_
        self.arr = [10**18] * (2*self.n-1)

    def update(self, k, a): #0_indexed
        k += self.n - 1
        self.arr[k] = a

        while k > 0:
            k = (k-1) // 2
            self.arr[k] = min(self.arr[2*k+1], self.arr[2*k+2])

    def query(self, l, r):
        L, R = l+self.n, r+self.n
        ide_ele = float("inf")
        #単位元(ide_ele)は、区間外の値などに設定する値です。
         #ex) 最小値のセグ木 → +inf
         #　和のセグ木 → 0
         #　積のセグ木 → 1
         #  gcdのセグ木 → 0
        res = ide_ele
        while L < R:
            if R & 1:
                R -= 1
                res = func(res, self.arr[R-1]) #

            if L & 1:
                res = func(res, self.arr[L-1])
                L += 1

            L >>= 1
            R >>= 1

        return res

n,m=map(int,input().split())
lrc=[tuple(map(int,input().split())) for _ in range(m)]
dp=SegmentTree(n+1)
dp.update(1,0)

rl=[[] for _ in range(n+1)]
rc=[[] for _ in range(n+1)]
for l,r,c in lrc:
    rl[r].append(l)
    rc[r].append(c)

for r in range(2,n+1):
    x=float("inf")
    for l,c in zip(rl[r],rc[r]):
        y=dp.query(l,r)+c
        if x>y:
            x=y
    dp.update(r,x)
    if r==n:
        ans=x

if ans==float("inf"):
    ans=-1
print(ans)