from collections import deque


N, K, *AB = map(int, open(0).read().split())

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


E = [[] for _ in range(N + 1)]
for a, b in zip(*[iter(AB)] * 2):
    E[a].append(b)
    E[b].append(a)


visited = [False] * (N + 1)

res = 0
Q = deque([1])
while Q:
    v = Q.popleft()
    visited[v] = True
    K -= 1
    if K == 0:
        break
    res += 1

    for u in E[v]:
        if not visited[u]:
            Q.append(u)

print(res)