function frwd()
  while not turtle.forward() do
    turtle.dig()
  end
end

function down()
  turtle.turnLeft()
  turtle.turnLeft()
  --turtle.digDown()
  --turtle.down()
end

function back()
  turtle.forward()
  turtle.digDown()
end

function hall()
  for i = 1, 21 do
    frwd()
  end
  down()
  for i = 1, 21 do
    back()
    if i%8 == 0 then
      turtle.select(16)
      turtle.placeDown()
    end
  end
end

function branch()
  turtle.turnLeft()
  hall(20)
  hall(20)
  turtle.turnRight()
end

function mine(blocks)
  for i = 1, blocks do
    frwd()
    if i%3 == 0 then
      branch()
    end
  end
  down()
  for i = 1, blocks do
    back()
  end
end

term.clear()
term.setCursorPos(1,1)
term.write("How far shall I dig?")
term.setCursorPos(1,2)
blocks = read()

mine(blocks)