import heapq

T = int(input())
N, M = map(int, input().split())
G = [dict() for _ in range(N)]
MAX = 10 ** 13

edges = []
for _ in range(M):
    u, v, w = map(int, input().split())
    u -= 1
    v -= 1
    edges.append((u, v, w))


def add_edge(u, v, w):
    G[u][v] = w
    if not T:
        # 無向グラフ
        G[v][u] = w


def dijkstra(source, target):
    dist = [MAX] * N
    d = [(0, source)]
    dist[source] = 0
    while d:
        c, v = heapq.heappop(d)
        if dist[v] < c:
            continue
        for t, cost in G[v].items():
            if dist[v] + cost < dist[t]:
                dist[t] = dist[v] + cost
                heapq.heappush(d, (dist[t], t))
    return dist[target]


for u, v, w in edges:
    add_edge(u, v, w)

ans = MAX
for u, v, w in edges:
    del G[u][v]
    if not T:
        # 無向グラフ
        del G[v][u]

    length = dijkstra(v, u) + w
    ans = min(length, ans)

    add_edge(u, v, w)
print(ans if ans != MAX else -1)