N, M = map(int, input().split())
LRC = sorted([list(map(int, input().split())) for i in range(M)], key=lambda x: (x[0], x[1]))

inf = float('inf')
dp = [inf] * N
dp[0] = 0

for l, r, c in LRC:
    for i in range(r, l, -1):
        if dp[i-1] > dp[l-1] + c:
            dp[i-1] = dp[l-1] + c
        else:
            break

if dp[-1] < inf:
    print(dp[-1])
else:
    print(-1)