# https://atcoder.jp/contests/abc014/submissions/3935971

class SegmentTree(object):

    __slots__ = ["elem_size", "tree", "default", "op"]

    def __init__(self, a: list, default: int, op):

        from math import ceil, log

        real_size = len(a)

        self.elem_size = elem_size = 1 << ceil(log(real_size, 2))

        self.tree = tree = [default] * (elem_size * 2)

        tree[elem_size:elem_size + real_size] = a

        self.default = default

        self.op = op

        for i in range(elem_size - 1, 0, -1):

            tree[i] = op(tree[i << 1], tree[(i << 1) + 1])



    def get_value(self, x: int, y: int) -> int:  # 半開区間

        l, r = x + self.elem_size, y + self.elem_size

        tree, result, op = self.tree, self.default, self.op

        while l < r:

            if l & 1:

                result = op(tree[l], result)

                l += 1

            if r & 1:

                r -= 1

                result = op(tree[r], result)

            l, r = l >> 1, r >> 1

        return result



    def set_value(self, i: int, value: int) -> None:

        k = self.elem_size + i

        self.tree[k] = value

        self.update(k)



    def update(self, i: int) -> None:

        op, tree = self.op, self.tree

        while i > 1:

            i >>= 1

            tree[i] = op(tree[i << 1], tree[(i << 1) + 1])



from operator import itemgetter

N, M = list(map(int, input().split()))

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

LRC.sort(key=itemgetter(0))



idx_LRC = 0

q = []



seg = SegmentTree([float("inf")] * (N+1), float("inf"), min)



seg.set_value(1, 0)

for v in range(1, N+1):

    d = seg.get_value(v, N+1)

    while idx_LRC < M:

        l, r, c = LRC[idx_LRC]

        if l <= v:

            seg.set_value(r, min(seg.tree[r+seg.elem_size], d + c))

            idx_LRC += 1

        else:

            break

ans = seg.tree[N+seg.elem_size]

print((ans if ans!=float("inf") else -1))

# https://atcoder.jp/contests/nikkei2019-2-qual/submissions/8357999 を

# 平方分割に書き換えた



class Rmq:

    # 平方分割

    # 値を変更すると元のリストの値も書き換わる

    def __init__(self, a, sqrt_n=500):

        from itertools import zip_longest

        self.n = len(a)

        self.sqrt_n = sqrt_n

        self.layer1 = a

        self.layer0 = [min(values) for values in zip_longest(*[iter(a)]*sqrt_n, fillvalue=float("inf"))]



    def get_min(self, l, r):

        sqrt_n = self.sqrt_n

        parent_l, parent_r = l//sqrt_n+1, (r+1)//sqrt_n

        if parent_l < parent_r:

            return min(min(self.layer0[parent_l:parent_r]),

                       min(self.layer1[l:parent_l*sqrt_n]),

                       min(self.layer1[parent_r*sqrt_n:r]))

        else:

            return min(self.layer1[l:r])



    def set_value(self, idx, val):

        self.layer1[idx] = val

        idx //= self.sqrt_n

        self.layer0[idx] = min(self.layer0[idx], val)



    def chmin(self, idx, val):

        if self.layer1[idx] > val:

            self.set_value(idx, val)



    def debug(self):

        print(("layer0=", self.layer0))

        print(("layer1=", self.layer1))



    def __getitem__(self, item):

        return self.layer1[item]



    def __setitem__(self, key, value):

        self.set_value(key, value)





from operator import itemgetter



N, M = list(map(int, input().split()))

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

LRC.sort(key=itemgetter(0))



idx_LRC = 0

q = []



rmq = Rmq([float("inf")] * (N+1))



rmq.set_value(1, 0)

for v in range(1, N+1):

    d = rmq.get_min(v, N+1)

    while idx_LRC < M:

        l, r, c = LRC[idx_LRC]

        if l <= v:

            rmq.chmin(r, d+c)

            idx_LRC += 1

        else:

            break

ans = rmq[N]

print((ans if ans != float("inf") else -1))

# https://atcoder.jp/contests/abc014/submissions/3935971
class SegmentTree(object):
    __slots__ = ["elem_size", "tree", "default", "op"]

    def __init__(self, a: list, default: int, op):
        from math import ceil, log

        real_size = len(a)
        self.elem_size = elem_size = 1 << ceil(log(real_size, 2))
        self.tree = tree = [default] * (elem_size * 2)
        tree[elem_size : elem_size + real_size] = a
        self.default = default
        self.op = op
        for i in range(elem_size - 1, 0, -1):
            tree[i] = op(tree[i << 1], tree[(i << 1) + 1])

    def get_value(self, x: int, y: int) -> int:  # 半開区間
        l, r = x + self.elem_size, y + self.elem_size
        tree, result, op = self.tree, self.default, self.op
        while l < r:
            if l & 1:
                result = op(tree[l], result)
                l += 1
            if r & 1:
                r -= 1
                result = op(tree[r], result)
            l, r = l >> 1, r >> 1
        return result

    def set_value(self, i: int, value: int) -> None:
        k = self.elem_size + i
        self.tree[k] = value
        self.update(k)

    def update(self, i: int) -> None:
        op, tree = self.op, self.tree
        while i > 1:
            i >>= 1
            tree[i] = op(tree[i << 1], tree[(i << 1) + 1])


from operator import itemgetter

N, M = list(map(int, input().split()))
LRC = [list(map(int, input().split())) for _ in range(M)]
LRC.sort(key=itemgetter(0))
idx_LRC = 0
q = []
seg = SegmentTree([float("inf")] * (N + 1), float("inf"), min)
seg.set_value(1, 0)
for v in range(1, N + 1):
    d = seg.get_value(v, N + 1)
    while idx_LRC < M:
        l, r, c = LRC[idx_LRC]
        if l <= v:
            seg.set_value(r, min(seg.tree[r + seg.elem_size], d + c))
            idx_LRC += 1
        else:
            break
ans = seg.tree[N + seg.elem_size]
print((ans if ans != float("inf") else -1))

# https://atcoder.jp/contests/nikkei2019-2-qual/submissions/8357999 を
# 平方分割に書き換えた
class Rmq:
    # 平方分割
    # 値を変更すると元のリストの値も書き換わる
    def __init__(self, a, sqrt_n=500):
        from itertools import zip_longest

        self.n = len(a)
        self.sqrt_n = sqrt_n
        self.layer1 = a
        self.layer0 = [
            min(values)
            for values in zip_longest(*[iter(a)] * sqrt_n, fillvalue=float("inf"))
        ]

    def get_min(self, l, r):
        sqrt_n = self.sqrt_n
        parent_l, parent_r = l // sqrt_n + 1, (r + 1) // sqrt_n
        if parent_l < parent_r:
            return min(
                min(self.layer0[parent_l:parent_r]),
                min(self.layer1[l : parent_l * sqrt_n]),
                min(self.layer1[parent_r * sqrt_n : r]),
            )
        else:
            return min(self.layer1[l:r])

    def set_value(self, idx, val):
        self.layer1[idx] = val
        idx //= self.sqrt_n
        self.layer0[idx] = min(self.layer0[idx], val)

    def chmin(self, idx, val):
        if self.layer1[idx] > val:
            self.set_value(idx, val)

    def debug(self):
        print(("layer0=", self.layer0))
        print(("layer1=", self.layer1))

    def __getitem__(self, item):
        return self.layer1[item]

    def __setitem__(self, key, value):
        self.set_value(key, value)


from operator import itemgetter

N, M = list(map(int, input().split()))
LRC = [list(map(int, input().split())) for _ in range(M)]
LRC.sort(key=itemgetter(0))
idx_LRC = 0
q = []
rmq = Rmq([float("inf")] * (N + 1))
rmq.set_value(1, 0)
for v in range(1, N + 1):
    d = rmq.get_min(v, N + 1)
    while idx_LRC < M:
        l, r, c = LRC[idx_LRC]
        if l <= v:
            rmq.chmin(r, d + c)
            idx_LRC += 1
        else:
            break
ans = rmq[N]
print((ans if ans != float("inf") else -1))

-# https://atcoder.jp/contests/abc014/submissions/3935971 -class SegmentTree(object): -    __slots__ = ["elem_size", "tree", "default", "op"] +# https://atcoder.jp/contests/nikkei2019-2-qual/submissions/8357999 を +# 平方分割に書き換えた +class Rmq: +    # 平方分割 +    # 値を変更すると元のリストの値も書き換わる +    def __init__(self, a, sqrt_n=500): +        from itertools import zip_longest -    def __init__(self, a: list, default: int, op): -        from math import ceil, log +        self.n = len(a) +        self.sqrt_n = sqrt_n +        self.layer1 = a +        self.layer0 = [ +            min(values) +            for values in zip_longest(*[iter(a)] * sqrt_n, fillvalue=float("inf")) +        ] -        real_size = len(a) -        self.elem_size = elem_size = 1 << ceil(log(real_size, 2)) -        self.tree = tree = [default] * (elem_size * 2) -        tree[elem_size : elem_size + real_size] = a -        self.default = default -        self.op = op -        for i in range(elem_size - 1, 0, -1): -            tree[i] = op(tree[i << 1], tree[(i << 1) + 1]) +    def get_min(self, l, r): +        sqrt_n = self.sqrt_n +        parent_l, parent_r = l // sqrt_n + 1, (r + 1) // sqrt_n +        if parent_l < parent_r: +            return min( +                min(self.layer0[parent_l:parent_r]), +                min(self.layer1[l : parent_l * sqrt_n]), +                min(self.layer1[parent_r * sqrt_n : r]), +            ) +        else: +            return min(self.layer1[l:r]) -    def get_value(self, x: int, y: int) -> int:  # 半開区間 -        l, r = x + self.elem_size, y + self.elem_size -        tree, result, op = self.tree, self.default, self.op -        while l < r: -            if l & 1: -                result = op(tree[l], result) -                l += 1 -            if r & 1: -                r -= 1 -                result = op(tree[r], result) -            l, r = l >> 1, r >> 1 -        return result +    def set_value(self, idx, val): +        self.layer1[idx] = val +        idx //= self.sqrt_n +        self.layer0[idx] = min(self.layer0[idx], val) -    def set_value(self, i: int, value: int) -> None: -        k = self.elem_size + i -        self.tree[k] = value -        self.update(k) +    def chmin(self, idx, val): +        if self.layer1[idx] > val: +            self.set_value(idx, val) -    def update(self, i: int) -> None: -        op, tree = self.op, self.tree -        while i > 1: -            i >>= 1 -            tree[i] = op(tree[i << 1], tree[(i << 1) + 1]) +    def debug(self): +        print(("layer0=", self.layer0)) +        print(("layer1=", self.layer1)) + +    def __getitem__(self, item): +        return self.layer1[item] + +    def __setitem__(self, key, value): +        self.set_value(key, value) -seg = SegmentTree([float("inf")] * (N + 1), float("inf"), min) -seg.set_value(1, 0) +rmq = Rmq([float("inf")] * (N + 1)) +rmq.set_value(1, 0) -    d = seg.get_value(v, N + 1) +    d = rmq.get_min(v, N + 1) -            seg.set_value(r, min(seg.tree[r + seg.elem_size], d + c)) +            rmq.chmin(r, d + c) -ans = seg.tree[N + seg.elem_size] +ans = rmq[N]