local n, k = io.read("*n", "*n")
local edge = {}
for i = 1, n do
  edge[i] = {}
end
for i = 1, n - 1 do
  local a, b = io.read("*n", "*n")
  edge[a][b], edge[b][a] = true, true
end
if n < k then
  print(-1)
else
  print(k - 1)
end