N, K = map(int, input().split())
G = [[] for i in range(N)]
for i in range(N - 1):
    a, b = map(int, input().split())
    a, b = a - 1, b - 1
    G[a].append(b)
    G[b].append(a)
    
if K >= N + 1:
    print(-1)
else:
    print(K - 1)