N, K = map(int, input().split())
ab = [[] for _ in range(N)]
ans = 0

for i in range(N-1):
    a, b = map(int, input().split())
    ab[a-1].append(b-1)
    ab[b-1].append(a-1)

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