local n, k = io.read("*n", "*n")
if n <= 2 * k then
  local reg = n - k
  local t = {}
  for i = 1, reg do
    table.insert(t, {i * 2 - 1, i * 2})
    table.insert(t, {i * 2 - 1, i * 2 - 1})
    table.insert(t, {i * 2, i * 2})
  end
  for i = reg * 2 + 1, n do
    table.insert(t, {i, i})
  end
  print(#t)
  for i = 1, #t do
    print(table.concat(t[i], " "))
  end
else
  print(-1)
end