N, K = gets.split.map(&:to_i)

G = Hash.new { |h, k| h[k] = [] }

(N - 1).times do
  a, b = gets.split.map(&:to_i)

  G[a] << b
  G[b] << a
end

if N < K
  puts -1
else
  puts K - 1
end