import sys
sys.setrecursionlimit(10 ** 6)

N, K = list(map(int, input().split()))
ab = []
for i in range(N-1):
    a, b = list(map(int, input().split()))
    ab.append([a-1, b-1])

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