from collections import deque

n, k = map(int, input().split())
x, y = [0] * (n - 1), [0] * (n - 1)
e = [[] for _ in range(n)]
for i in range(n - 1):
    x[i], y[i] = map(int, input().split())
    x[i] -= 1
    y[i] -= 1
    e[x[i]].append(y[i])
    e[y[i]].append(x[i])
# n<=k　と 単純に深さの和を足せば良いbfs
if k > n:
    print(-1)
else:
    print(k - 1)