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

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