import numpy as np
import scipy.sparse.csgraph
n,k=map(int,input().split())
a=np.array([[0]*n for i in range(n)])
for i in range(n-1):
    b,c=map(int,input().split())
    a[b-1][c-1]=1
d=k-sum(i==1 for i in scipy.sparse.csgraph.shortest_path(a,indices=0))
print(-1 if d>=2 else d)