from sys import setrecursionlimit
setrecursionlimit(10**5)
n,k=map(int,input().split())
g=[[] for _ in range(n)]
for _ in range(n-1):
    a,b=map(int,input().split())
    g[a-1].append(b-1)
    g[b-1].append(a-1)
d=[-1]*n
d[0]=1
def dfs(now,pre):
    for to in g[now]:
        if to==pre:
            continue
        d[to]=d[now]+1
        dfs(to,now)
dfs(0,-1)
if max(d)>=k:
    print(k-1)
else:
    print(-1)