import heapq


class Dijkstra:

    def __init__(self, rote_map, start_point, goal_point=None):
        self.rote_map = rote_map
        self.start_point = start_point
        self.goal_point = goal_point

    def execute(self):
        num_of_city = len(self.rote_map)
        dist = [float("inf") for _ in range(num_of_city)]
        prev = [float("inf") for _ in range(num_of_city)]

        dist[self.start_point] = 0
        heap_q = []
        heapq.heappush(heap_q, (0, self.start_point))
        while len(heap_q) != 0:
            prev_cost, src = heapq.heappop(heap_q)

            if dist[src] < prev_cost:
                continue

            for dest, cost in self.rote_map[src].items():
                if cost != float("inf") and dist[dest] > dist[src] + cost:
                    dist[dest] = dist[src] + cost
                    heapq.heappush(heap_q, (dist[dest], dest))
                    prev[dest] = src
        if self.goal_point is not None:
            return self.get_path(self.goal_point, prev)
        else:
            return dist

    def get_path(self, goal, prev):
        path = [goal]
        dest = goal

        while prev[dest] != float("inf"):
            path.append(prev[dest])
            dest = prev[dest]

        return list(reversed(path))

n,m=map(int, input().split()) 
route_map = [dict() for _ in range(n)]

for i in range(m):
    u, v, a = map(int, input().split())
    u, v = u - 1, v - 1
    route_map[u][v] = a
    
for i in range(n-1):
    u, v = i+1, i
    route_map[u][v] = 0
    

# ゴールまでの最短経路
dijkstra_result = Dijkstra(route_map,0).execute()


if dijkstra_result[-1]!=float('inf'):
    print(dijkstra_result[-1])

else:
    print(-1)


    
    
    
    
        
    

 






        
    
    
    
    
    
        
    

 






        
        


        
    
    
    
    
    
        
    

 






        
    
    
    
    
    
        
    

 






        
        


        
    
    
    
    
    
        
    

 





