import sys
import numpy as np
from scipy.sparse.csgraph import dijkstra
from scipy.sparse import csr_matrix

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

T = int(readline())
N, M = map(int, readline().split())
frm, to, cost = np.array(read().split(), np.int64).reshape(-1, 3).T

G = csr_matrix((cost, (frm, to)), (N + 1, N + 1), dtype=np.float64)

ans = np.inf
for u, v, c in zip(frm, to, cost):
    G[u, v] = np.inf
    x = c + dijkstra(G, directed=T, indices=[v])[0, u]
    ans = min(ans, x)
    G[u, v] = c

if ans == np.inf:
    ans = -1
else:
    ans = int(ans + .5)
print(ans)