n,k=map(int,input().split())
if k>n: print(-1);exit()
elif k==1: print(0);exit()
edges=[[] for _ in range(n)]
for _ in range(n-1):
    a,b=map(int,input().split())
    a-=1;b-=1
    edges[a].append(b);edges[b].append(a)
q=[0]
visited=[False]*n
k-=1;cnt=0
while q:
    v=q.pop(0)
    for e in edges[v]:
        if visited[e]: continue
        visited[e]=True
        k-=1;cnt+=1
        if k==0:
            print(cnt);exit()
        q.append(e)
print(-1)