import sys
input = sys.stdin.readline
sys.setrecursionlimit(10 ** 7)

N, M = map(int, input().split())
E = []
for i in range(M):
    L, R, C = map(int, input().split())
    E.append([L-1, R-1, C])

E = sorted(E, key=lambda X: X[1])

# N_はN以上の2べき数
N_ = 2**(len(bin(N))-2)
INF = 10**10 # 10**10 
seg = [INF] * (2*N_-1)

# k番目(0index)をaに更新
def update(k, a):
    k += N_-1
    seg[k] = a

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

# 開区間！！！[a, b)の最小値を求める
def query(a, b, k, l, r):
    if r <= a or b <= l:
        return INF
    if a <= l and r <= b:
        return seg[k]
    else:
        vl = query(a, b, k*2+1, l, (l+r)//2)
        vr = query(a, b, k*2+2, (l+r)//2, r)
        return min(vl, vr)



# 初期化
update(0, 0)

for i in range(M):
    new = query(E[i][0], E[i][1], 0, 0, N_) + E[i][2]
    if new < seg[E[i][1] + N_-1]:
        update(E[i][1], new)

if seg[N-1+N_-1] == INF:
    print(-1)
else:
    print(seg[N-1+N_-1])