import sys
from collections import defaultdict as dd
input = sys.stdin.readline

N, M = map(int, input().split())
d = []


# N: 処理する区間の長さ

INF = 2**31-1

LV = (N).bit_length()
N0 = 2**LV
data = [INF]*(2*N0)
lazy = [None]*(2*N0)

# 伝搬対象の区間を求める
def gindex(l, r):
    L = (l + N0) >> 1; R = (r + N0) >> 1
    lc = 0 if l & 1 else (L & -L).bit_length()
    rc = 0 if r & 1 else (R & -R).bit_length()
    for i in range(LV):
        if rc <= i:
            yield R
        if L < R and lc <= i:
            yield L
        L >>= 1; R >>= 1

# 遅延伝搬処理
def propagates(*ids):
    for i in reversed(ids):
        v = lazy[i-1]
        if v is None:
            continue
        lazy[2*i-1] = data[2*i-1] = lazy[2*i] = data[2*i] = v
        lazy[i-1] = None

def query(l, r):
    propagates(*gindex(l, r))
    L = N0 + l; R = N0 + r

    s = INF
    while L < R:
        if R & 1:
            R -= 1
            s = min(s, data[R-1])
        if L & 1:
            s = min(s, data[L-1])
            L += 1
        L >>= 1; R >>= 1
    return s

# 区間[l, r)をxで更新
def update(l, r, x):
    *ids, = gindex(l, r)
    propagates(*ids)
    f = 0
    if l > 1:
      f = query(l, r)
    #print(f, l, r)
    L = N0 + l; R = N0 + r
    while L < R:
        if R & 1:
            R -= 1
            yl = INF
            if lazy[R-1] != None: yl = lazy[R-1]
            lazy[R-1] = data[R-1] = min(x + f, data[R-1] + x,yl)
        if L & 1:
            yl = INF
            if lazy[L-1] != None: yl = lazy[L-1]
            lazy[L-1] = data[L-1] = min(x + f, data[L-1] + x,yl)
            L += 1
        L >>= 1; R >>= 1
    #print(data)
    for i in ids:
        data[i-1] = min(data[2*i-1], data[2*i])


for _ in range(M):
  d.append(list(map(int, input().split())))
d.sort(key = lambda x: (x[0], x[1]))
#print(d)
for i in range(M):
  update(d[i][0], d[i][1] + 1, d[i][2])
  #print(d[i])
  #print(data)

res = query(N, N + 1)
if res != INF: print(res)
else: print(-1)
