import collections
N,K = map(int,input().split())
lsg = [[] for i in range(N)]
for i in range(N-1):
    a,b = map(int,input().split())
    a -= 1
    b -= 1
    lsg[a].append(b)
    lsg[b].append(a)
#できるかどうかの判定が必要
depth = [-1]*(N)
depth[0] = 0
d = collections.deque()
d.append(0)
while d:
    v = d.pop()
    for j in lsg[v]:
        if depth[j] >= 0:
            continue
        depth[j] = depth[v]+1
        d.append(j)
lsc = [-1]*(N)
def dfs(k):
    cnt = 1
    for j in lsg[k]:
        if depth[j] < depth[k]:
            continue
        cnt += dfs(j)
    lsc[k] = cnt
    return cnt
dfs(0)

print(-1 if N < K else K-1)