import sys


class AlgSegmentTreeMin:
    def __init__(self, n, default_value):
        self.Nelem = n
        self.default_value = default_value
        self.size = 1 << (n.bit_length())
        self.data = [self.default_value] * (2 * self.size)

    def build(self, raw_data):
        # raw_data は 0-indexed
        for i, x in enumerate([self.default_value] * (self.Nelem + 1)):
            self.data[self.size + i] = x
        for i in range(self.size - 1, 0, -1):
            x = self.data[i + i];
            y = self.data[i + i + 1]
            self.data[i] = x if x < y else y

    def update(self, i, x):
        i += self.size
        self.data[i] = x
        i >>= 1
        while i:
            x = self.data[i + i]
            y = self.data[i + i + 1]
            self.data[i] = min(x, y)
            i >>= 1

    def get_value(self, left, right):
        left += self.size
        right += self.size + 1
        x = self.default_value
        while left < right:
            if left & 1:
                y = self.data[left]
                x = min(x, y)
                left += 1
            if right & 1:
                right -= 1
                y = self.data[right]
                x = min(x, y)
            left >>= 1
            right >>= 1
        return x


def solve():
    readline = sys.stdin.buffer.readline
    mod = 10 ** 9 + 7
    n, m = list(map(int, readline().split()))
    stree = AlgSegmentTreeMin(n, 10 ** 15)
    stree.build()
    lrc = [list(map(int, readline().split())) for _ in range(m)]
    lrc.sort()
    stree.update(0, 0)
    for l, r, c in lrc:
        l -= 1
        r -= 1
        tm = stree.get_value(l, r)
        now = stree.get_value(r, r)
        tm = tm + c if tm + c < now else now
        stree.update(r, tm)
    mt = stree.get_value(n-1, n)
    print(mt if mt != stree.default_value else -1)


if __name__ == '__main__':
    solve()


### FIXED CODE ###

import sys


class SegTreeMin:

    def __init__(self, n, max_v):
        self.n = n
        n2 = 1
        while n2 < n:
            n2 <<= 1
        self.n2 = n2
        self.tree = [max_v] * (n2 << 1)
        self.INF = max_v

    def update(self, i, x):
        i += self.n2
        self.tree[i] = x
        while i > 1:
            self.tree[i >> 1] = x = min(x, self.tree[i ^ 1])
            i >>= 1

    def get_min(self, a, b):
        result = self.INF
        q = [(1, 0, self.n2)]
        while q:
            k, l, r = q.pop()

            if a <= l and r <= b:
                result = min(result, self.tree[k])
                continue

            m = (l + r) // 2
            k <<= 1
            if a < m and l < b:
                q.append((k, l, m))
            if a < r and l < m:
                q.append((k + 1, m, r))

        return result


def solve():
    input = sys.stdin.readline
    mod = 10 ** 9 + 7
    n, m = list(map(int, input().rstrip('\n').split()))
    seg_tree = SegTreeMin(n, 10 ** 15)
    seg_tree.update(0, 0)
    lrc = [list(map(int, input().rstrip('\n').split())) for _ in range(m)]
    lrc.sort()
    for l, r, c in lrc:
        l -= 1
        r -= 1
        tm = seg_tree.get_min(l, r)
        seg_tree.update(r, min(tm + c, seg_tree.get_min(r, r + 1)))
    mt = seg_tree.get_min(n-1, n)
    print(mt if mt != 10 ** 15 else -1)


if __name__ == '__main__':
    solve()