n, k = map(int, input().split())
es = [[] for _ in range(n)]
for i in range(n-1):
    a, b = map(int, input().split())
    es[a-1].append(b-1)
    es[b-1].append(a-1)

if n < k:
    print(-1)
    exit()

print(k-1)