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:
class SegTree:
    def __init__(self, N):
        self.N = N
        self._N = 1<<((N-1).bit_length())
        self.node = [10**15]*(2*self._N-1)
        self.node[self._N-1] = 0

    def eval(self,p):
        i = p + self._N-1
        cur = self.node[i]
        while i:
            i = (i-1) // 2
            if cur > self.node[i]:
                cur = self.node[i]
        self.node[p+self._N-1] = cur
        return cur

    def update(self,x,p,q,idx=0,a=0,b=None):
        if not idx:
            x += self.eval(p)
        if x >= 10**15:
            return
        if b is None:
            b = self._N
        if q <= p or b <= p or q <= a:
            return
        elif p <= a and b <= q:
            if x < self.node[idx]:
                self.node[idx] = x
            return
        else:
            self.update(x,p,q,idx*2+1,a,(a+b)//2)
            self.update(x,p,q,idx*2+2,(a+b)//2,b)
            return


n,m = map(int, input().split())
edge = [tuple(map(int, input().split())) for _ in range(m)]
edge.sort()
tr = SegTree(n)
for s,t,c in edge:
    tr.update(c,s-1,t)
ans = tr.eval(n-1)
if ans >= 10**15:
    ans = -1
print(ans)