N, K = map(int, input().split())
G = [[] for _ in range(N)]
for _ in range(N-1):
    a, b = map(int, input().split())
    a-=1
    b-=1
    G[a].append(b)
    G[b].append(a)
if N<K:
    print(-1)
    exit()
print(K-1)