from collections import deque
import sys

N, M = map(int, input().split())
d = [set() for _ in range(N+1)]
for _ in range(N-1):
    a, b = map(int, input().split())
    d[a].add(b)
    d[b].add(a)

queue=deque([1])
vs = set([1])
vs_bfs = list()
parents = [0] * (N+1)
dist = [0] * (N+1)
while queue:
    v = queue.popleft()
    vs_bfs.append(v)
    for u in d[v]:
        if u in vs:
            continue
        parents[u] = v
        vs.add(u)
        queue.append(u)
        dist[u] = dist[v] + 1

if N < M:
    print(-1)
else:
    print(M-1)