N, K = map(int, input().split())
G = [list() for _ in range(N)]
for i in range(N - 1):
    u, v = map(int, input().split())
    u -= 1
    v -= 1
    G[u].append(v)
    G[v].append(u)

if N < K:
    print(-1)
else:
    print(K - 1)